Mary Portas has, without a hint of warning, just launched into song.

Specifically the 1999 pop song Keep on Movin' by boyband Five. Portas's head is swaying in time to the music. "What's the one when they sing? You know...." she asks her business partner Peter Cross, who is at this moment furiously searching the internet on his phone for the answer. She sings in a surprisingly soulful voice: "Get on up when you're down... ."

It's hard to tell where the rest of the room – which includes about four PR people, a store manager, a photo-grapher, her civil partner Melanie Rickey and Cross – stand on this issue, but I'm tempted to join in.

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That's the thing about Portas: her enthusiasm is infectious. You see it in the television series she has fronted – Mary Queen of Shops, Mary Queen of Charity Shops, Mary Portas: Secret Shopper, Mary Queen of Frocks and Mary's Bottom Line. You see it in the projects she gets involved in (The Portas Review, the government-commissioned high street review, and the Living & Giving charity shop concept for Save the Children, which includes an outlet in Edinburgh) and you can see it – feel it in fact – in this wood-panelled director's room in House of Fraser in Glasgow. She's a one woman show, a veritable force of nature and it's hard not to get caught up in it all.

Portas is in Frasers to launch her latest Mary womenswear and homewear shop (the only one in Scotland) but that's not the only thing on her mind. The singing, for instance, was inspired by a conversation about the Portas Pilots, the town regeneration initiative that was launched as a result of The Portas Review. So far almost 400 towns have completed their video entries to become one of the 12 towns which will share more than £1 million of government funding. Cornwall town Helston's offering showed a selection of residents miming along to Five's Keep on Movin' hit. "Those videos will make your heart leap. There's a sense of this is my country and this is how bad this has got. It's just the most phenomenal thing."

She feels equally enthusiastic about her Glasgow store. "It's stunning isn't it?" she asks rhetorically. Portas says she decided to open in the city because "there's a real great aesthetic here, a heritage around design and fashion".

The Mary & House of Fraser collection is a bright, bold affair full of neon-coloured separates and jersey dresses. The collection is designed, according to Portas, with older women in mind. The youth-focused Topshop or Warehouse this is not – these clothes are made for women in their 40s and 50s by a woman who knows all about dressing at that age.

"I suppose the big thing I found when you hit 50 is the clothes went into the neutrals: beige on beige with a bit more beige," she says, explaining why she decided to set up her own fashion label.

"I'm so not a beige person and I really wanted to represent so many wonderful great women – to give them the opportunity to actually express themselves through clothes in a way that reflected their modernity and where they were in their lives. I'm 51 and I feel fantastic – I'm at my peak – and I want clothes that do that, and I don't want to be told [to wear beige] or be put into clothes that girls who are 18 should be in, because actually at your age that's not the way you should be going."

The inspiration for Portas's collection came from her own changing room experiences. While her signature look (razor sharp bright orange bob, leather biker jackets, bold colours and take-your-eye-out enormous necklaces) has now been honed, in part thanks to Rickey who is the fashion editor at large of Grazia magazine, Portas hasn't always been so sartorially confident.

"When I hit 40 I went into the fitting rooms at Harvey Nichols and it was that time when all the boho lacy Chloe was coming through and I remember thinking that's what's in this season, that's what I've got to wear. It was just the most horrible experience because I'm very linear, very sharp, but I was still trailing onto this look. It was after I'd had children [Portas has two children, Mylo and Verity, to her ex-husband Graham Portas] and it was at that time when you're at your most vulnerable."

Portas, who launched retail and brand communication agency Yellowdoor in 1997, isn't the kind of woman who would let a lacy designer frock beat her though.

"I went home and I thought: no I'm not paying this for this – this isn't me and I'm not doing this. My partner is in the fashion industry, and she's got an incredibly brilliant eye, and so we pared down what I absolutely loved in my wardrobe and then I thought: that's going to be my look. So I made sure I stuck with what was absolutely me. I totally kept that look."

The fashion lessons Portas learned have been transferred into her own collection. The mannequins, which come complete with sharp Irn-Bru coloured bobs, might look like a plastic Portas army but according to the woman herself there's something in there for everyone. You just have to follow the rules in "Mary's Manifesto". "Don't buy loads of cheap, don't buy into every single trend. Women do that as they get older because they don't want to feel old – they think if I'm not trendy I'm old so they still keep doing it."

Portas has, in her own words, "never been a shrinking violet" though that's not the whole picture. She's an intelligent woman – sharp, quick-witted and always on the ball with whatever she's discussing, but there's a softer side too.

It was evident in her latest Channel 4 programme Mary's Bottom Line, which was about her new underwear label Kinky Knickers. During the series Portas spent time with the previously unemployed young apprentices who would eventually make her underwear range. As she heard their stories the camera would show tears glistening in her eyes. These weren't fake, just-for-the-ratings TV tears though, it was real wobbly chin stuff – the kind of tears that prove your heart is really breaking.

Portas's commitment to the Kinky Knickers project is as strong now as it was when the cameras were rolling. "We've got seven new trainees. I'm searching for other factories, and trying to see how we can expand and develop. I'd really love to sell [the Kinky Knickers products] to China, it would be good to be selling our British stuff back to them."

Perhaps surprisingly one of the key sections of Portas's House of Fraser shop has nothing to do with clothes. The homewear collection, which contains British-made artisan products, has been handpicked by Portas – apparently one of her favourite jobs. "I love interior design, I may be a bit more into that than [fashion]. I adore it. To me being in a place that makes your heart sing is a very sensual experience. Home to me is crucial and being around lovely things."

British manufacturing and British high streets: these are two of the things Portas feels most passionate about. These are the things she says she wants to support and make better. "I really believe in change, I think people can. I think we underestimate the power of the human spirit. The way that I've lived my life is what's the next thing, what am I onto now? I see myself as a kid thinking what's my next thing."

She is glad that fame came to her later in life, when she already knew who she was as a person. "I think it was Oprah [Winfrey] that said fame defines you, if you don't know who you are it defines you. But I don't think it defines me."

Determination is something that defines Portas. Since filing that government high street report has she considered running for parliament? The eyebrows are raised.

"No, thank you, it's too political. I would get extraordinarily frustrated. You'd turn up and you'd think why can't you just make that happen – I don't want to [hear that], just get it done. You go into that environment and it's diffused and I hate diffusion – I like clear thought and to keep punching."