This is the first general election in nearly a century where the internal cohesion and even the viability of the United Kingdom itself have been seriously on the agenda. Small groups of Scottish, Welsh and Irish nationalists have sat at Westminster for more than 40 years. But the prospect of the election of a large number of Scottish nationalist MPs dedicated to independence, with real leverage, would be a new situation, albeit with some echoes of Ireland’s impact in late 19th-century politics.

Even nationalists have been caught on the hop at the speed of events, and have said contradictory things – a few of them at the SNP’s manifesto launch on Monday – about how they might behave if the polls are proved right. The rest of Britain is even less prepared, and many pro-union politicians have been thoughtless and provocative in their responses too. It is an extremely volatile moment. British politics on all sides needs to raise its game. So do the British people.

There is little sign of games being raised as the general election draws nearer. The SNP manifesto launch was done with professionalism, but in at least three very important ways its is an opportunist document. The first of these is separatism. The election is not about independence, Nicola Sturgeon said on Monday, yet independence is what the SNP exists to achieve. Ms Sturgeon’s protestations of friendship towards the rest of the UK may be sincere, but her failure to rule out a future independence referendum in the lifetime of the next parliament inevitably means other parties have to be wary about SNP promises.

The second problem is fiscal devolution in the aftermath of last year’s “vow” to Scotland. Until only a few days ago, the SNP was strenuously arguing the case for full fiscal autonomy – Scotland raising all its own income and making all its own spending decisions with the exception of defence and foreign affairs. Critics who challenged the sums – which, according to the Institute of Fiscal Studies, would leave a £7.6bn hole in Scotland’s finances that would require brutal spending decisions at Holyrood – were mocked as fainthearts or told the figures did not matter. Now, in the manifesto, full fiscal autonomy has become “the transition to full fiscal responsibility”. Such slipperiness may not alarm those committed to the SNP as a matter of nationalist faith. But it makes the SNP difficult to rely on for anyone else.

The third issue is Scottish votes on English and British matters. Until recently, the SNP has stood aside at Westminster on such votes, partly because the issues are devolved to Scotland and partly as a nationalist identity stance. On Monday, however, the SNP unveiled a list of UK tax and spending proposals and a shopping list of priorities for the next UK government on which it offers to give or withhold support at Westminster. This inevitably and deliberately stirs the West Lothian question – whether Scottish MPs should vote on issues for England when English MPs cannot vote on those issues for Scotland – and creates further uncertainty.

The Conservatives have been as guilty as the SNP in fanning the mix of Scottish provocation and English irritation

And not just that, but resentment too. The Conservatives have been every bit as guilty as the SNP in fanning the current mix of Scottish provocation and English irritation. David Cameron is in many ways as big a threat to the union as Ms Sturgeon, as he proved so disastrously after the referendum in 2014. It has certainly come to something when Michael Forsyth, the Thatcherite former Tory Scottish secretary, has to warn Mr Cameron to stop inflaming the Scots, as we report . But the variable political geometry of post-devolution Britain – the needs of Wales and Northern Ireland matter as much as those of England or Scotland – is nevertheless an issue that can no longer be ignored.

Those problems are better solved together than separately, in this union as in the European one. But there is much work to be done on mending and modernising Britain’s – and Europe’s – institutions if they are not to stand in the way of a better union between the UK’s different nations. A constitutional convention, as promised by Labour, may be part of the solution, though the party does not show enough enthusiasm for the idea. The big question facing us all is whether nationalism is the 21st century’s best answer to Britain’s social and economic problems. The equally big answer is that it is not.

• This article was amended on Tuesday 21 April 2015 to make a change to the standfirst.