The Democratic Unionist Party yields to no one, it hardly needs to be said, in its loyalty to the United Kingdom.

The colours of its lion-head logo are unambiguously red, white and blue. It is represented at every level of political life, from the Lords to local government. In a partnership (of sorts) with Sinn Fein, it governs a significant part of the UK.

This is news, it turns out, to broadcasters struggling to cajole David Cameron into pre-election TV debates. While the BBC and ITV prepare to extend invitations to the Scottish Nationalists, the Greens, Ukip and Plaid Cymru, Westminster's fourth largest party can, it seems, be safely ignored.

Peter Robinson, the DUP's leader, is furious, predictably enough. Fury is what he does best. But in this instance the First Minister of Northern Ireland has a point, one that will not be lost on voters who return eight MPs to Westminster, the biggest group of MLAs to the Northern Ireland Assembly, and a host of local councillors. Is this how they are regarded by the UK they defend?

Pretty much. Already, the intention to exclude the DUP from the debates is being presented in London as a simple reflection of political reality. Unlike, say, the SNP, Mr Robinson's party is somehow not pertinent to the main General Election event. The politics of Northern Ireland is, supposedly, an internal matter for the province.

Factually, that isn't true. The Tories continue to make attempts, with no success whatever, to establish themselves at Stormont, but they do exist in Northern Ireland. Ukip meanwhile has an MLA, one David McNarry, and a handful of councillors. So why should those voices be echoed in two of three proposed TV debates, but not the voice of the DUP?

Attempting to unpick the logic in all of this is as good a way as any of deciphering metropolitan attitudes in an altered political landscape. Mr Cameron's insistence that the Greens must be involved in the debates might have been a transparent attempt to dodge the cameras. Inadvertently, however, the Prime Minister has flushed out broadcasters who could not - or would not - grasp the extent to which things have changed across the UK.

Take Mr Robinson's point. Why should Westminster's fourth-biggest party be excluded after Ukip's Nigel Farage was given the TV nod with only opinion polls to go on? The SNP is about to make waves, but by what right do broadcasters prejudge a party's likely importance? The Greens might be gathering support, but they have only a single MP. And in what world does Plaid count for more than the DUP? Or is it simply assumed that if you invite one Nationalist party you must invite both?

The Liberal Democrats are unhappy, meanwhile, at being excluded from the final "head to head" between Mr Cameron and Ed Miliband. Comically, this vaunted "party of government" resents being made to look small. It also argues that a 7-7-2 arrangement for the three debates pencilled in for April is fast becoming a mess. As it happens, that's perfectly true. But the fact arises because broadcasters have recognised, belatedly, that we are heading for a glorious mess at the General Election.

Return to Mr Robinson and his fury. He is well aware that Nicola Sturgeon will have her TV moment - if Mr Cameron does not renege again - because the SNP could well hold the balance of Westminster power after the polls close in May. The DUP leader has his response ready: two can play at that game. It turns out, in fact, that Mr Robinson, leader of a profoundly right-wing party, one wooed relentlessly by Mr Cameron, would have no problem doing "business" with Labour. There is, says the DUP man, "no question" about that.

Officially, this would amount to no more than the sort of confidence and supply arrangement envisaged, for now, by Ms Sturgeon. But at least one unnamed DUP MP told the Daily Telegraph the other day that "we wouldn't rule out a more formal coalition". Mr Miliband, visiting Northern Ireland, meanwhile spent most of a morning with Nigel Dodds, the DUP's Westminster group leader.

As ever, Labour's only admitted goal is an overall majority. You could wonder, nevertheless, about the reaction from potential voters to the signals being exchanged with the late Ian Paisley's old party. Just a precaution against possible Labour losses in Scotland? If that's the case, a little clarity on the DUP's price for co-operation would be handy.

Defence spending, for one, might be an interesting test for Mr Miliband. As a percentage of GDP it is predicted to fall again this year, but Mr Robinson and his patriotic party want it raised to a Nato-decreed two per cent. Would Labour really deliver that in these straitened times for the sake of a deal with the DUP?

Come to that, would Mr Miliband - or Jim Murphy - explain to Scottish voters why Labour does not intend to oppose the devolution of corporation tax to Northern Ireland? With the concept of "detriment" in tax matters becoming an issue for Scotland as new powers are loaned to Holyrood, Mr Miliband said in Belfast, for the first time, that he will not stand against the concession offered by George Osborne to Stormont, but not - certainly not - to Edinburgh. So how does that work for the newly "patriotic" Scottish Labour?

TV debates have become a cipher for the horse-trading that has already commenced. The DUP, reasonably enough, is not prepared to be excluded from either ritual. All parties with an axe worth grinding are busy talking of "issue by issue" Westminster votes, but fundamental questions loom ever larger as a political system fragments.

First, there is the question of who can form a government. Secondly, tantalisingly, there is the issue of how a government might be sustained. The SNP will have no truck with the Tories, but might arrange something of mutual benefit with Labour. Would that be Labour with the DUP in tow, or does Mr Miliband believe eight MPs from Northern Ireland could liberate him from the Nationalist vice?

It seems reasonable to assume, equally, that if Labour is considering doing "business" with Mr Dodds and his group, hopes of Lib-Lab deals are fading. Nick Clegg and his friends seem to have enjoyed their time in coalition more than some Liberal Democrats and "progressive" consensus-builders want to believe. A future Tory understanding with Ukip could be entered in the same ledger, whatever Mr Cameron says.

The fat fly in this ointment remains the fact that no one knows how the General Election will play out. Mr Miliband's new interest in Northern Ireland might count as nothing more than a bit of insurance, for use only in a dire emergency. The SNP might find it impossible to work with a wounded and resentful Labour Party. Those risible TV debates might meanwhile transform fortunes in unsuspected ways.

The sheer unpredictability of the 2015 General Election is one obvious symptom of profound change in British political life. Those we used to call big parties hate the fact. That counts as a welcome result before a single vote has been cast.