The less things change, the happier David Cameron is... and on Scottish independence and Britain's membership of the EU, it is no different

In polls since January 2013, Scotland's 'Yes' vote has only gained about 1 per cent

David Cameron's greatest achievement as Prime Minister could be to keep things as they are. If he keeps Scotland in the United Kingdom and the UK in the European Union, he will have done a good job, not just in his own eyes – he is a conservative, after all – but in the eyes of most Labour supporters.

Indeed, there are far more people in the Conservative Party than in Labour who would be happy for Scotland to become independent, and for England (with Wales and Northern Ireland, presumably) to become independent of the EU. They would be pleased for a partisan reason if Scotland left, believing that the Tories would win elections ever after. An overlapping group of Tories would be even more pleased, for a nationalist reason, if England left the EU. The two objectives combined under a banner "England Alone".

Both groups of Tories are mistaken. The Tories-for-Salmond tendency make the mistake of assuming that there is a lump of Labour votes in the UK. They think that, if they could get rid of the sizeable chunk of Labour votes in Scotland, where there are hardly any Tory votes, this would tip the balance in England and Wales in their favour for ever and a day.

They really do have short memories. In 1997, Labour won more votes than the Conservatives in England: 44 per cent to 34 per cent. In 2001, Labour still had a six-point lead. Even in 2005, Tony Blair's third election, the Conservatives managed to get ahead by only 35.7 per cent to 35.5 per cent. The Labour Party has elected leaders capable of appealing to the English centre ground before, and it can do so again.

Oddly, the Tories who want to leave the EU fall for a different lump of labour fallacy. The Better Off Outers tend to believe that, if the UK restricted the entry of workers from the rest of Europe, there would be more and better-paid jobs for British workers, and it would be easier to get British people off benefits and into work. This is the fallacy known to economists that there is a fixed amount of work in an economy, and that restricting immigration, or limiting the hours people are allowed to work, would create more jobs. It seems like common sense – the slogan painted on the side of Ukip's purple battle bus – but it doesn't work. Open, flexible labour markets create more work, and more demand, which creates more jobs.

There, then, is the paradox of Cameron. Only a Conservative prime minister was ever likely to have to fight a referendum on Scottish independence, because a Tory government in Westminster is Alex Salmond's boo-bear scare story. The Scottish National Party is not covert about this calculation. Nicola Sturgeon, Salmond's deputy, last week responded to the Prime Minister's speech thus: "David Cameron, as the Tory prime minister, is the very embodiment of the democratic case for a 'Yes' vote for an independent Scotland – and he knows it."

It is a desperate ploy from a party that I think expects to lose the referendum in September. Much was made last week of recent opinion polls that were said to show support for independence closing the gap on the "No" vote. But if you look at all the polls since the beginning of last year, average "Yes" support has gained about one percentage point in 12 months. At this rate, Salmond might win a referendum in 2024.

The only thing that might swing the campaign is the promise of a second referendum after terms of separation are negotiated: in focus groups, that option reassures those who are hesitant about taking an irrevocable step. But it is too late for Salmond to go for that, and Cameron would, in any case, be under no pressure to agree to it.

Furthermore, only a Conservative prime minister is likely to be able to win a referendum to keep Britain in the EU. If Ed Miliband were to win next year's election, my guess is that it would be after he matched Cameron's promise of a referendum. And a Labour government would be quite likely to lose it. It would not be as tough in negotiating better terms, and so would not have as good a case as Cameron might for claiming to have secured the British national interest. No wonder the Prime Minister is laying on the full bunting and solemnities for Angela Merkel, the German Chancellor, when she visits London later this month. Contrast the Grade I listed pomp accorded to her with the half pint in a pub near the airfield for Miliband's ami, François Hollande, at the end of last month. Merkel is the one with the power. The other reason a Labour government might lose a referendum is that the Conservative Party in opposition would become fiercely anti-EU and fight the campaign as an anti-government cause.

There is more at stake at the next election than Cameron's attempt to avoid being remembered as a one-term stopgap. If he wins, he could be a 10-year prime minister who saved the Union, kept Britain in Europe and who thus defended, against a large minority of his own party, objects in which Miliband believes even more passionately than he.