The possible breakup of the country barely impinged on most British voters during the 2019 general election campaign. For the majority, Brexit was overwhelmingly the dominant issue. But Boris Johnson’s refusal this week to allow Nicola Sturgeon’s Scottish government to hold a second independence referendum is a reminder of one of the Brexit election’s most important and umbilically linked consequences.

In Scotland things were, of course, different during the election. Yet even here it is important to recognise that Brexit, not the future of the union, was also at the front of the stage. In the Scottish National party’s 2019 manifesto, Sturgeon is pictured at a rostrum with a single slogan: Stop Brexit. Polling showed two Scottish voters in three thought Brexit a key issue, many more than chose any other subject. No one who followed the campaign in Scotland can seriously dispute that the SNP’s tremendous success on 12 December, when it took 48 of the 59 seats in Scotland, owed at least as much to its opposition to Brexit as to the issue of independence.

But that was then, and this is now. A second Scottish referendum – inescapably abbreviated to indyref2 – is again at the centre of British politics. In the week after the election, buoyed by the SNP’s success, Sturgeon demanded the power to hold another vote – which legally rests with the UK government. On Tuesday, Johnson wrote formally to refuse it. The two are in a standoff. But this is not a private battle of mandates. The breaking up of Britain is an issue with which all parties, politicians and citizens will need to wrestle.

One of the consequences of ignoring a subject for too long is that once people are forced to engage, they can adopt and speak up for stances that don’t withstand scrutiny. That’s a live danger in this debate, especially for English politicians who can all too easily find themselves feeding nationalist grievances by making ignorant and condescending remarks.

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This certainly applies to many English Conservatives, who have suddenly rediscovered their unionism but without having thought what 21st-century unionism might look like. Johnson is not at all the worst offender. In practice he focuses, as he did in his letter to Sturgeon and did again in the Commons ,on Wednesday on the very usable argument that the SNP is trying to reopen something it said in 2014 was a once-in-a-generation decision. Others, though, are less sensitive.

Uncompromising, London-centric unionism is an arrogant and stupid look. It won’t cut it – as Irish history, as well as common sense, should remind them.

Yet an equivalent danger applies to Labour. It contrived not to mention the words United Kingdom in its 107-page election manifesto. But not all its leadership candidates have been so coy. Rebecca Long-Bailey has attacked Labour’s participation in the successful 2014 Better Together campaign. Meanwhile Jess Phillips said on Wednesday she would have done what Johnson did. Sturgeon, she said, had “no mandate” for indyref2 because the SNP’s 45% vote share in Scotland did not constitute a majority, even though it had delivered a landslide in seats. This is dry political tinder for a Scottish Labour party that was almost wiped out in December.

Play Video Nicola Sturgeon requests Scottish independence referendum powers – video

The political realities of the indyref2 argument are rather more nuanced and complex than some of these confrontations suggest. To listen to the kind of exchanges that took place at prime minister’s questions on Wednesday between Johnson and the SNP’s Westminster leader, Ian Blackford, you might get the impression that this battle is coming to a head very soon. Actually that’s not the case. Many on all sides admit that this is a bit of a phoney war.

Both sides are digging in for a long battle of attrition rather than a quick, decisive contest. Sturgeon knows she has little chance of moving Johnson’s position without a significant change in the political weather – and maybe not even then. That doesn’t mean she will dial down the indignation. On the contrary. The more hyperbolic the language – the Scottish finance secretary, Derek Mackay, described UK ministers as Scotland’s “imperial masters” this week – the clearer it becomes that the SNP is playing to the gallery and not expecting an early independence vote.

Sturgeon is permanently under pressure from her militants to ramp up the tactics. Scotland’s constitutional affairs secretary, Mike Russell, claimed on Wednesdaythat the SNP had “many options”. That may be true. But his problem is that few of them offer much prospect of success.

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Sturgeon has not ruled out legal action – but lawyers, including the party’s most prominent lawyer, Joanna Cherry, say it is unlikely to override Johnson. Some flirt with an SNP boycott of the Commons; but this tactic, with its echo of Sinn Féin, might alienate moderate opinion. The so-called Catalan option – an illegal referendum without UK authority – has repeatedly been ruled out by Sturgeon. The Scottish parliament will certainly be asked to renew its call for indyref2, but this will be a gesture. That leaves Sturgeon with the electoral route, in which the SNP tries to win a fresh mandate in the May 2021 Holyrood elections and then petitions the UK government once more.

Even if this happens, Tory sources do not think Johnson will change his stance. Time, they argue, is on the prime minister’s side, not Sturgeon’s. The longer this standoff continues, the harder it could get for the SNP – in office for 13 years and counting – to persuade voters that independence is the uniquely effective answer to Scotland’s problems in the NHS, schools and transport. That’s without even mentioning the unknowable impact of Alex Salmond’s trial in March, whatever its outcome.

Counterintuitively, given Scotland’s emphatic vote to remain in the EU in 2016, time could also be on Johnson’s side because of Brexit. Like it or not, Brexit will change the terms on which any independence referendum would be fought. Britain, including Scotland, will be out. If Scotland tried to rejoin after independence, issues such as the currency, borders and the UK single market would rise quickly up the agenda.

Time might even be good for the Scottish Labour party, if only on the grounds that it is hard to see things getting worse for it. Yet in the next few years it will be Johnson who holds the UK key, not Jeremy Corbyn’s successor. Johnson does not want to be the prime minister who won Brexit but lost Britain. If he is serious about bringing Britain closer together, he cannot rely simply on defying the SNP. He will also have to make Britain work for Scotland. He will have to use direct UK government spending in Scotland on things like infrastructure, hospitals and education. That may be far from his mind right now. If this happens, Scotland’s Life of Brian question – what has the UK ever done for us? – might start to get a surprising answer.

• Martin Kettle is a Guardian columnist