A few weeks ago, Ken Loach welcomed an unusual visitor to his offices in London: Angelina Jolie.

It turns out the Hollywood star and torchbearer for humanitarian causes is a big fan of the veteran British director. So, with security guards flanking her, she popped by for tea one afternoon. It's a delicious thought: a tete-a-tete between the left-wing Loach and Brad Pitt's other half. "She just wanted to say hello," says Loach, with typical modesty, "so she came round and said hello. She was very nice to talk to."

With his hard-hitting social realist films, Loach is the antithesis of the brand of Hollywood glamour ordinarily associated with Jolie. In fact, with his grey hair and spectacles, this humble 75-year-old barely fits in with the media professionals who swan around Soho, where his production company Sixteen Films is based. He's more at home away from London – be it the Midlands, where he was born and raised; Northampton, where he first directed repertory plays in 1961; or Glasgow, the city that has become like a cinematic second home for him for the past 16 years.

Today, he's ensconced in the cutting room, working on a new documentary, The Spirit Of 45, which deals with the rise of socialism in the UK in the wake of the Second World War. "I'm finding it harder than I thought it was going to be," he says, all-too aware he hasn't made a non-fiction film since his 1998 Liverpool dockers' strike documentary The Flickering Flame. But we're here to talk about The Angels' Share, his latest Scottish excursion, one that offers a more upbeat mood than films like My Name Is Joe and Sweet Sixteen.

If Loach is a little distracted, it's partly because the film is about to play in the Cannes Film Festival, where his 2006 film The Wind That Shakes The Barley won the Palme d'Or. A festival favourite, Loach has had 11 films play in competition – a remarkable record for a British filmmaker. But does he still get nervous presenting his films there? "Absolutely," he nods. "You're unveiling it for the first time. You've got nothing to go on. But you've just got to hope that it's not going to be too bad – that you're going to get away with it again."

At least The Angels' Share should raise a smile. An earthy tale, its central character is Robbie (Paul Brannigan), who is, on the surface, a typical ned. An early scene – one of the most powerful Loach has filmed – sees Robbie confronted by a young man he brutally attacked in the street, leaving him blind in one eye. While such an unsympathetic character could easily alienate the audience, Loach gradually wins you over after Robbie is forced to undertake community service.

"We wanted to take the audience on quite a long journey, and because Robbie has got a dark past, and has done some terrible things with terrible consequences. We wanted to risk alienating the audience from it, but hoping the audience would want him to succeed in the end. It seemed a bigger challenge, rather than having him a sweet young lad all the way through. You want to take the audience on that journey of seeing somebody doing something despicable but still wanting him to get through at the end."

As for how Loach manages it, the clue is in the title, which refers to the small percentage of whisky that evaporates during the maturation process. After being introduced to the pleasures of the drink by his community service supervisor, who takes him to a distillery and a tasting, Robbie and his friends plan a good-natured heist that could set them up for life when they get wind of an upcoming whisky auction. If it's not quite Ken-Loach-does-Ealing-comedy – it's no modern-day Whisky Galore! – it has the same grassroots humour so much of his work uses to soften its blows.

"You could take the same characters and tell a tragedy or a comedy," says Loach, who sees this film mark his 10th outing with screenwriter Paul Laverty, a rock-solid partnership that began with Carla's Song in 1996, in which Robert Carlyle's Glasgow bus driver becomes embroiled with a Nicaraguan exile. Loach cites Laverty, whose father is Scottish and who trained as a lawyer in Glasgow, as the main reason why he's returned so often to Scotland to shoot. "Paul is from the west coast of Scotland, so that's his idiom. And I learned very early on, you work where the writer writes most easily."

While Loach and Laverty haven't always stuck to this – Bread And Roses, for example, focused on a janitors' strike in Los Angeles – unquestionably their best work has been set around Glasgow, a city Loach seems to love. "Glasgow has the same quality Liverpool has," he says. "It's a very strong working-class culture, built out of political and social struggle. It's had big industries which have faded away, so there's a memory of all that entails. Because the culture that grew out of the shipbuilding, the humour is strong and the language is sharp. It's a vivid culture and the people have a lot of energy."

Even if VisitScotland might not always thank him (at least The Angel's Share takes in the Balblair distillery near Tain), Loach seems ideally placed to assess the mood of the country. In particular, the continuing call for Scottish independence. "If I had the chance to be independent from the Tory-Liberal-New Labour bunch, I'd jump at it," he says, with a bittersweet laugh. "[If there's] any way we can get English independence from the current bunch of politicians, we should all embrace it."

As for the referendum, Loach is all for it. "Scotland has the right to hold any referendum it likes," he says. "The English ruling class are such dyed-in-the-wool imperialists that they can't conceive anything can happen without their approval. But I think: go for it. Other colonised countries have asserted their independence."

Surprisingly, given his passion for politics, Loach's left-wing beliefs did not stem from his upbringing. His father, an electrician-turned-foreman at a factory in Coventry, was "non-political and a Daily Express reader" when Loach was a student, reading law at Oxford. It was only after his stint directing plays in Northampton that Loach truly became a political animal, when he joined the BBC in 1963. "It was a very political time, a time of the occupations and events in Paris in May 1968 and the [Harold] Wilson government, which people quickly got disillusioned with. It was very much on the general agenda then."

Loach forged early crucial partnerships during his time at the BBC, notably with producer Tony Garnett, with whom he made a series of TV plays, documentaries and films, including the celebrated 1969 venture Kes. But for the next two decades Loach saw his career stall, his work either dogged by poor distribution or censorship, notably when, in 1971, he and Garnett were commissioned by Save The Children to make a film. Executives were appalled when the resulting work was deeply critical of the organisation and wanted the negative destroyed. After tough negotiations, one was kept in the BFI vaults, on condition it was not shown unless sanctioned by the charity. It eventually screened for the first time last September.

In 1982, Loach made Questions Of Leadership, a documentary commissioned by Channel 4 about the trade unions under the shadow of Thatcherism. That too was shelved (it was eventually shown 12 months later).

Loach's fuse is lit when I bring up the name of Sir Jeremy Isaacs, the former Channel 4 chief executive responsible. Ironically, Isaacs later accused Loach of an act of censorship when, in 2009, the director called for a boycott of the Edinburgh International Film Festival after organisers received a donation of state money from Israel to support the travel costs of a short film director coming over from Tel Aviv.

The festival handed the money back and Isaacs lashed out at Loach. "They must not allow someone who has no real position, no rock to stand on, to interfere with their programming," he said.

Loach laughs when I bring this up."He's in no position to talk about censorship. He better take a step back on that one. In fact, I'd love to meet him in public to debate censorship."

Loach is keen to stress that his support of the cultural boycott of Israel is not aimed at any individuals but against the intervention by the state. "It's a very important subject," he says. "It's the great cause of our time, the struggle for Palestinian liberation. We've got to be strong on the boycott, because it's requested by Palestinians who are suffering by having their land taken away and houses destroyed and people killed, and being imprisoned in their own country. So I would argue strongly for the boycott of state-sponsored Israeli work."

If a conversation with Loach can often feel like taking the weight of the world on your shoulders, there's a lot of joy there too. Take his discoveries. His best films over the past 20 years have seen him plunder the wealth of Scottish acting talent. Robert Carlyle's first major film role was in Loach's 1991 film Riff-Raff, set around a London building site. Likewise Peter Mullan made an early appearance in the same film, and would go on to win Best Actor in Cannes for his heartbreaking portrayal of an alcoholic in My Name Is Joe. Other actors launched through Loach include Atta Yaqub (who made his debut in Ae Fond Kiss) and Martin Compston (ditto, Sweet Sixteen).

So is it simply that Scotland is overflowing with acting talent? "It is," Loach says, nodding. "It is awash with great performers. You meet them as soon as you get off the train, as you walk through Central Station. You see performers all over the place." In the case of The Angels' Share, Loach went back to William Ruane and Gary Maitland – both of whom made their debuts in Sweet Sixteen (and reunited with Compston for Loach's contribution to 2005 anthology film Tickets, as a trio of Rome-bound Celtic fans).

In the case of Brannigan, raised in the east end of Glasgow, it seems Loach has unearthed another rough diamond. With a past that echoes Robbie's own upbringing, Brannigan spent time in Polmont young offenders institution after he was charged with "discharging a firearm" – the result of a long-running feud. "He's not had an easy life," says Loach. "He's been in some tough situations." Laverty met Brannigan while he was working in a Glasgow community centre, then persuaded him to meet Loach. It was worth it: Brannigan delivers an instinctive, blistering performance.

While The Angels' Share might be optimistic in tone, there's no doubting the underlying message: the majority of young people face bleak employment prospects. "It's desperate," says Loach. "The worse thing is you see young people with no possibility of work, and the work that is there - It's short-term agency work, it's no-hours contracts where you're guaranteed no work at all. It's casualised. There's no way they can build a career or even establish a skill that would define them. And with that definition comes dignity and self-respect and the capacity to plan your life."

From Kes onwards, children and teenagers have often featured at the heart of Loach's work, and there seems to be a strong paternal streak in him. With his wife Lesley, with whom he will celebrate his 50th wedding anniversary in July, Loach fathered five children (his second eldest, Nicholas, died in a road accident in 1971 when he was six years old).

"My kids are grown up now," he says, "but I worry for their children. Absolutely. I can't see how people are going to find a place to lead a decent, dignified life."

Two of Loach's offspring have followed him into film – Emma makes documentaries, while Jim has gone from directing episodes of Coronation Street to making his feature debut with 2010's Oranges And Sunshine.

While he was promoting the film, I asked Jim what the effects were on the Loach children, having a father who directed films. "We weren't always on set or traipsing around the country," he replied. "In our teenage years, we were brought up in Bath, so it was a pretty normal childhood." Assuming you consider it normal to spend Christmas Day afternoon helping prepare the pitch of Bath City FC – where Loach is now a shareholder after a fans' buyout – ahead of the Boxing Day match.

It seems fitting that Loach would support a non-league club, the football equivalent of his own homegrown films, because he has rarely enjoyed success at the box office. Even 2009's Looking For Eric, in which Eric Cantona played himself, flopped outwith Manchester.

Recently, Prime Minister David Cameron suggested British filmmakers and producers should aim to make "commercially successful pictures".

What does Loach make of Cameron's statement? "Cameron's an idiot when it comes to films," he replies. "I was going to say he should stick to what he knows about but that's not much."

Taking himself out of the picture for a minute, does Loach think Scottish filmmakers should be creating more commercial fare – a Celtic equivalent of Four Weddings And A Funeral, perhaps? "I think what we need is a real diversity," he says. "You need a system that enables people's creativity to express itself. And that means you have a diverse range of films. The problem is we don't have that diversity – the multiplexes are dominated by either American films or films the Americans like, that fit their ideas. That's the problem. There's always a great range of people with different ideas. The problem is they can't get into the cinemas and they can't get their films made."

It's probably no surprise that Loach has never been nominated for an Oscar (unlike Mike Leigh, the British director he has often – rather falsely – been compared to).

Neither has he ever won a Bafta, despite being nominated several times – an omission that stems less from his filmmaking talent and more from his ability to get up the noses of the establishment. And make no mistake – that will continue. If you're educated, he says, "you have an obligation to deal with things". He may be of an age when most people slow down in life, but his conscience won't let him rest. n

The Angels' Share (certificate TBC) screens at the Cannes Film Festival on Tuesday and opens in the UK on June 1.