Labour has spent the last week gurning about how unlucky it is with the media, the political weather and much else. But now comes proof that Lady Luck favours no one: it was not, despite the claims of some Tories, anything other than bad luck for David Cameron that the Irish vote on Europe coincided with the start of his conference. And so, here we go, back to the future.

It seems half a lifetime ago that Westminster was boiling with abstruse-sounding rows about Maastricht, sovereignty and weighted voting systems, with Tory MPs hurling abuse at each other. Yet with the Irish vote removing the last major obstacle to the Lisbon treaty, it's hard to see how David Cameron can avoid returning to the blood-soaked old battlefield. The Czechs and the Poles haven't signed up yet, but yesterday the Czech prime minister, Jan Fischer, said he believed his country would ratify the treaty by the end of the year. Cameron's hopes that the treaty could be delayed until after the British general election look extremely slim.

So the question facing the Tory leadership is quite clear: if, by next May, the Lisbon treaty has come into force and Europe has a new president, quite possibly Tony Blair, will Cameron keep his promise to hold a referendum? Yes or no? It's a straightforward question. He knows that to do so would risk a huge row with the rest of Europe, and a fully operational treaty would be harder to unpick than one not yet signed. That's why until now he has used the weaselly words that, if the treaty is signed, he would "not let matters rest there".

Cameron also knows that many in his party, not least his would-be successor Boris Johnson, will push for a referendum and have the support of much of the media too. If Cameron appears to want to renege on his promise, he will provoke fury and rebellion on his own side. For now, his "wait and see" gambit is beginning to look indecisive. If he were Gordon Brown, he would undoubtedly be accused of dithering.

At the same time, Cameron is worrying about another referendum, one which may prove no less momentous for the future shape of Britain. He faces a two-sided constitutional struggle, looking south towards Europe – but also north towards the Scots.

The nightmare for Cameron is that, once George Osborne has revealed details of the cuts imposed by Tory Westminster on Scottish budgets, the SNP start to gain momentum for their proposed independence referendum. Alex Salmond, Scotland's first minister and nobody's fool, has been watching the Conservative agendas on cuts and on Europe with fascination.

His strategy for a referendum will be fleshed out on 30 November, St Andrew's Day, and it's likely he will try to get the vote in the spring or autumn of 2010, presumably when news about public spending cuts is causing maximum anger. It isn't easy for the SNP in Edinburgh's "parliament of minorities" because, although they are the largest party, there is a clear Labour-Lib Dem-Tory majority against a referendum.

But things aren't as simple as they seem. For one thing, the SNP are well ahead of Labour in the polls, while the Tories' situation remains dire. Cameron may be cleaning up across England, and even in Wales; but he cannot expect more than one or two MPs from Scotland. His plan for cutting the number of Westminster seats to 500 will inevitably further weaken the Scottish link.

So the argument about independence and the SNP referendum will take place at a time when Labour in Scotland is horribly weak, and when the Tories themselves seem to lack legitimacy there. Salmond has been menacing about the likely consequences for these parties if they go into an election for the Edinburgh parliament in 2011 as the politicians who refuse to let the people of Scotland speak. It's a volatile situation.

The way Salmond describes his future Scotland is carefully calculated. He has been wooing the financiers and banks to rebuild Scotland's place as a financial centre. And he speaks, again and again, about the importance to Scotland of Europe.

This is crucial because it connects to the Tories' coming war against European federalism. As Cameron, William Hague and the others get into a battle over the constitution and the future of Europe, the Scottish government will be offering itself as a pro-European bastion, just as the Irish did – and nobody knows better than Salmond what a huge financial benefit that once won for Dublin. Many Tories will say, of course, that all this is absolutely fine. According to them, the Scots have been a revenue-sapping bunch of whingers for years, whose main export to England seems to have been politicians and journalists. An independent Scotland means a Tory majority in England way into the distant future. And it makes standing up to the EU easier, in many ways, because Eurosceptic opinion is particularly strong in England. What's the problem?

Well, according to those who know him well, Cameron sees this as a definite problem. I'm told he loathes the idea of being the last ever prime minister of the United Kingdom. He would see the loss of Scotland as a huge blow to Britain's status in the world, including inside the EU and Nato, and would fight very hard to stop it happening. Last Friday, as a first step, he told the Scottish Sun he would "govern Scotland with the respect it deserves" and promised regular visits to answer questions.

Cameron is surely right to be concerned. If the prospect of an all-out confrontation with the rest of the EU is unsettling to middle of the road opinion, the end of the UK is much more so. What do you call the country that remains? It isn't England, quite, because there is also Wales. Does it stay a Diminished Britain, a Little Britain, whose flag is a simple spider of red lines on white? Trident, of course, goes because the naval bases in Scotland go. What about the currency? If the euro is circulating just north of Newcastle and Carlisle, the pound will feel more embattled.

Hardly anybody in England seems to be talking about this. There is little about it scheduled for the fringes and mainstream speeches at Manchester. Compared to public spending cuts and the deficit, it may seem marginal. But if you want a glimpse into how different life might feel under Cameron, and what an unfamiliar country we may be tiptoeing into, the very future of a UK inside Europe is a good place to start.