The official opening of the Olympics is almost upon us. A senior figure in UK athletics reckons that Great Britain is sending “the best prepared and arguably strongest” track and field team ever. Mo Farah and Jessica Ennis-Hill will represent the reawoken spirit of London 2012, and newer names will presumably make their mark. By rights, the prospects of British athletes should be easily filling the high summer news vacuum – and yet so far, there seems to be a weird sense of national detachment.

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Of course, there is a world of difference between a Games held in London, and the demand to focus domestic attention on Rio de Janeiro. But still: aside from all that controversy about Russian doping, the media seems to be going through the motions, as if the public might be beyond rousing. In my local Sainsbury’s, three or four racks of Team GB T-shirts lie apparently untouched and unloved.

Even with people who usually need no persuasion to turn the conversation to sport, I have yet to have a single conversation about what’s going to happen. Yes, loud passions will presumably be stirred when medals start to be won – but there again, our strange, punch-drunk national mood might soon place the Olympics as another event that was distractedly absorbed before the next calamity came round the corner, and the now-normal feelings of angst and gloom resumed.

When I clapped eyes on those T-shirts they sparked a strange kind of double-take, as if they were reminding me of something I’d forgotten: Great Britain, remember? And so it was that my mind went back four years, to union flags wrapped around triumphant athletes, and Danny Boyle’s joyous tribute to the industrial revolution, the NHS, James Bond and the Sex Pistols. 2012 was hardly yesteryear: Alex Salmond confirmed the looming Scottish referendum, the Leveson report was published, and David Cameron started his third year in Downing Street. So how come it feels so distant?

Facebook Twitter Pinterest Mo Farah wins the mens 5000m during the Muller Anniversary Games at the Queen Elizabeth Olympic Park on 23 July. ‘When I clapped eyes on those T-shirts they sparked a strange kind of double-take, as if they were reminding me of something I’d forgotten: Great Britain, remember?’ Photograph: Stephen Pond - British Athletics/British Athletics via Getty Imag

The answer is pretty simple. Scotland is now on a fast track to a second independence referendum, and secession is a racing certainty – and in any case, that country is a separate political entity already, bar all but the formalities. The word England now seems to stand for the disaffected, angry, self-consciously English multitudes who voted to leave the EU – for whom “English” means “not London” and not middle class, either.

In the most populous parts of Wales, support for Brexit – and Ukip – suggests the same mood, although Cardiff is seen as the crucible of a modern Welsh identity, and at the same time, figures in the ruling Labour party have floated the idea of splitting from their English comrades. Meanwhile, Northern Ireland delicately maintains peace and power sharing, while the question of its eventual geopolitical destiny remains unresolved. All the time the terms “United Kingdom” and “Great Britain” seem to mean less and less. Amid all those widening cracks, how do you find even the outline of a coherent country, let alone any sense of shared purpose?

If you doubt any of this, think not just of the Boyle spectacular, but of the last occasion a senior politician tried to put flesh on Britain’s decaying bones. “Britishness”, as you may recall, was used by Gordon Brown as a failed means of giving himself a political theology, dealing with the SNP menace and trying to still rising English furies. “A strong sense of being British helps unite and unify us,” he said in early 2007. “It builds stronger social cohesion among communities … I’m very proud of being British; proud of British values, proud of what we contribute to the world.”

Yet the game was already up. Four years later, at the 2011 census, 60% of people in England identified as “English” only. In Wales, 66% thought of themselves as Welsh – and only about 10% of those people also considered themselves British. The same year’s census in Scotland found that 62% of people there identified as exclusively Scottish. Brown had talked about building “an even stronger sense of national purpose which unifies us for the years to come”, a sentiment so out of time that it looks almost absurd: two months after he spoke, the SNP took power in Scotland, and Britain’s great fracturing intensified, at speed.

This story is as much cultural as political, and we have all lived it. In my teens Britain and the UK were largely uncontested concepts: devolution was over a decade away, and even in the Tory-voting Cheshire suburbs, I barely heard anyone talk about England. But hindsight suggests that in my 20s, the great hype of Cool Britannia and Britpop – along with Tony Blair’s faux patriotic claims to be creating a “young country” – represented Blighty’s last cultural stand, of which the 2012 opening ceremony was a kind of final reprise.

Now, I try to explain some idea of Britain to my seven year-old daughter, and can’t quite manage. Scotland, she knows: it’s the place that had to choose between yes and no, where her tartan T-shirt came from. Wales is a cinch: she was born there, she supported its football team through the chain of wonders at Euro 2016, and it’s where her extended family are from. England is where she lives: the country that defines not just our small patch of the West Country, but the thriving, cosmopolitan place down the road in Bristol. But Britain? Sooner or later, I may have to bow to the inevitable and play her Billy Bragg’s prophetic Take Down the Union Jack, which bluntly cuts to the quick: “Britain isn’t cool you know, its really not that great / It’s not a proper country, it doesn’t even have a patron saint / It’s just an economic union that’s passed its sell-by date.”

It really has. It’s also a fading signifier for pretensions to global importance that Brexit has decisively done for. It’s the name of the nonexistent empire referred to in the titles of all those dodgy honours. It’s somewhere cited by politicians – witness Michael Howard’s “British dream”, Ed Miliband’s “British promise”, and David Cameron’s tributes to something he called “the British spirit” – whose words fail to ring true, not just because patriotism of the Westminster variety is usually a put-on, but because the imagined country to which they refer barely exists anymore.

In that sense, ironically enough, there is probably every reason to cheer on Team GB with even more passion than we all mustered in 2012. By 2020 Scotland will probably be gone, and England and Wales will be tied into even more impossible knots. Fly the flag and then fold it away: rarely has the very British notion of the last hurrah been more appropriate.