mong the saddest figures to appear on a platform beside the returning officer early on Friday morning was the candidate for the Braveheart Party, who called himself William Wallace and stood in Edinburgh Central. He was dressed in a kilt with a plaid wound about his chest, and his face was painted with woad on one side. When the returning officer announced he had won 191 votes, he came forward and roared, as Mel Gibson had in the film from which his party took its name. "They may take our lives, but they will never take our fre-e-edo-o-om!" Someone in the audience clapped and cheered briefly, and the rest of the platform, including the candidate for the Scottish National Party, shuffled.

He was sad, but also useful. For he had done the service of pushing the fantasy a little too far and in doing so, revealing its tawdriness.

Braveheart was a film which, for a while, seemed to have captivated Scotland. Football and rugby teams viewed it before big matches to give them spirit. Alec Salmond, the SNP leader, regards it as his favourite film. But, in seeking to animate it for the Scottish parliamentary elections, the kilted, woaded fantasist made the film appear the tawdry thing it was.

Most nations weave some sort of myth about themselves. It is often religion married to expansion - as the British one, notoriously, still is. "Send her victorious" and "Wider still and wider/Shall thy bounds be set" - many of us cannot bear to hear, let alone sing, these ludicrous boasts now, but they are still the official and quasi-official anthems. Scotland's myths were in some ways even more powerful (and certainly more fervently believed still) for being myths about a nation shorn of a state. The teeming imagination of Sir Walter Scott gave us much of "traditional" Scotland seen through a romantic nationalist haze; the 20th century added harder edges, sketching in a kind of proletarian nationalist sentiment centred on the Clyde.

Neither was overtly political in the sense of proposing separation; that was left to the late-1960s and 1970s, when the SNP managed to fuse cultural with political nationalism as Labour and Conservative governments floundered, and then grew further in the Eighties as a Thatcher government cut hard against the corporatist Scots political culture. Braveheart was the florid and debased apogee of that - positing the Scot as the noble and tormented victim of an epicene yet sadistic race. From where we are now, it was also, perhaps, the high point for nationalists.

On one count, that would not seem to be so. For the Scots and Welsh elections have seen the nationalists advance, especially in Wales where they had been largely trapped in the Welsh-speaking north and west. Now they are the second party - as they will be in the Scots parliament, even if they did not (as once seemed possible) displace Labour from its first position.

Yet there is a paradox here. Both have achieved their positions at the expense of fundamental nationalism - especially in Wales, where Plaid Cymru has more or less explicitly dropped its call for an independent Wales in favour of being the party which is the keenest to make the devolved parliament work. It also, like the SNP, tends to pitch itself a little to the left of Labour, hoping to capitalise on political cultures more leftist than the English, more wedded to state spending (which tends to be more generous in both areas).

It may be that we are seeing emerge not so much a nationalist challenge as a cultural accommodation. The collapse of the old form of unionism - which was expressed through such sentiments as are contained in the national anthem, or "Rule, Britannia" - has released the always-strong cultural identities of Wales and Scotland from an apolitical thrall, and encouraged the formation of politics and parties round those cultures.

There is presently no way of telling whether, as the nationalists claim, this is the end of the UK or its strengthening in diversity, as Labour says. The answer will lie in daily practice; in the perceived competence of the representatives; in the state of the national and regional economies; on the popularity of the government. We cannot know the outcome of these largely fortuitous elements but we do know that neither outcome is pre-ordained.

I hope, as a unionist, that Labour is right (indeed, this New Labour belief is the only hope unionists now have). It seems more possible than it did some years ago. The advance of quasi-nationalism at least makes explicit and open that we have a raft of identities - Scots, Welsh, British, European - even, if you wish, global. It is no longer a bad joke to suppose that one can be a gay Labour Scots Muslim Pakistani British European - indeed, there must be some people who fit that description.

If, as New Labour increasingly proclaims, its ideal is a cosmopolitan, multicultural society, the task of policy is to allow multiple identities to live together easily within individuals as well as between them. Some of these attributes will be more linked to tradition than others - as, in the example given, Muslim and Pakistani. But attributes like Scottish, or European, can be taken up as a matter of right (and of responsibility); Labour as a matter of political choice; sexual orientation as a matter of nature and of self-expression. The truly civic state gives all of these room to breathe; one of the reasons why Britishness is precious is that it has had a civic rather than an ethnic basis to it since its inception in the 18th century.

When one of these identities is pushed or insisted on as being the essential one - that is, when one is held to be Scottish above all, or gay above all - then the others suffer. When pushed to a limit, it becomes extremism. Modern liberals are more and more constrained to defend cosmopolitanism, and to rally people round open and multicultural societies in which the guidelines of tradition have ceased to operate; indeed, so strong is this pull that both the Welsh and Scots nationalists have themselves had to present themselves as liberal pluralist parties, careful to insist that Welshness or Scottishness does not depend on speaking Welsh or being called Mac. The more they achieve success, the more they must assure their voters that Scottishness of Welshness is a mirror image of Britishness - and the less they become "pure" parties.

It would be an irony indeed, but not an impossible one, if the Scots and Welsh nationalists assisted the development of a multicultural and diverse Britain, rather than dismembered it. There are some signs they are having precisely that effect; the speeches of Gordon Brown in the past month, as he brings himself to address the issue of Britishness, show a politician rising to the challenge of defining a new kind of union which does not need to be made mightier yet, but does need to be made more open. Celtic nationalism has gloried, and glories this weekend, in disturbing New Labour; in the longer run, it may be caught in its modernising, cosmopolitan coils.