This is the sound of British politics. A crescendo rumble, a deafening crash, a four-letter word. The blond skateboard king from behind the cycle shed, his shirt-tails flapping, has fallen off yet again. Unfortunately, the cracked pavement under his wheels is called Britain, or optimistically the United Kingdom, and he and his mates have been pounding it to destruction for more than three years.

In that time, the Brexit ordeal has changed Britain. Not as much as some think. Many of these changes, above all the English sense of powerlessness and resentment of elites, were already gathering speed 10 years ago, as Europe and the world crawled out of the banking disaster. Brexit disputes only accelerated them. The 2016 referendum didn’t so much create new divisions within England as rediscover old ones, especially in its aftermath. It was almost laughable that so few remain voters knew a leaver, or vice versa. England is still a country astonishingly segregated by class, by location, by attitudes towards power and privilege. Before the last war ended, the doomed fighter pilot Richard Hillary asked: “Was there perhaps a new race of Englishmen arising out of this war, a harmonious synthesis of the governing class and the great rest of England?” No, there wasn’t.

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One sinister change, still developing, is the breakdown of Britain’s immunity against lies. From about 2010 onwards, Conservative leaders began to blame the public spending of previous Labour governments for the huge financial deficit caused by the bank crash. What was most appalling was not the barefaced untruth. It was that nobody seemed to have the energy to rebut that lie. Millions of people were left to believe it, and probably still do. The Brexit campaign released an avalanche of feebly contested lies, on which the present prime minister still happily slaloms.

Within England, political geography has changed. More accurately, the way people see the enormous gap in wealth, opportunity, infrastructure and innovation between the London region and “England outside London” has sharpened. The “northern powerhouse” efforts, and HS2, only revive a long history of sticking-plaster remedies for what were once called “distressed areas”.

The change now is that two streams of bitter resentment – at London’s unfair wealth and privilege and at Westminster’s unfair refusal to “obey the people” – have converged into a single torrent. London is the most spectacularly diverse metropolis in Europe, which even after Brexit will go on sucking in the money of global oligarchs and hedge funds, and the location of new British institutions. London as an independent city state, like Singapore, would prosper, but in the short term it will bankrupt whatever is left of Hillary’s “great rest of England”.

It’s commonly said that the Brexit years have made the English more xenophobic, less tolerant, more angrily divided among themselves. The first is clearly true. Non-British Europeans confirm a new nastiness, even just a new coldness. So, even more emphatically, will migrants from Somalia, Nigeria, India, Bangladesh.

It’s true that English dislike of foreigners is ancient, abating in the middle 19th century as the public welcomed political refugees from failed revolutions, but only for an interval. A recent example: in a sample of adults in Scotland who list Pakistani as their ethnicity, 31% identified as Scottish. The corresponding sample taken in England found only 15% identifying as English. That tells you nothing particularly wonderful about Scotland, except that it’s a normal country. But it does tell a tale about England: a nation self-consciously unlike others and uneasy about sharing its “essence” with others.

More angry, more violent in the course of these three years? Very angry, certainly, but no angrier (and so far much less violent) than their great-grandparents were over Irish home rule or rights for women. Or, come to that, than some of their fathers and mothers were over the miners’ strike or the poll tax. It’s remarkable that in the past three years it’s the remainer half of Britain that goes roaring down the streets. The leavers, with their own powerful case to feel aggrieved and betrayed, stay at home and share their wrath with family and friends. England has a vigorous history of rioting. But for this generation, the default leave position seems to be: “Leave us in peace! Get it done, and go away.” There is much talk about the need to reform Britain’s democracy, but nothing like any coherent idea of what should replace it.

At the core of the three-year wrangle is constitution fuzziness. Where does final authority rest? Is it with the people, speaking through a referendum? Or with a prime minister claiming in a spooky way that he embodies the Queen and her prerogative ? Or with the doctrine of parliamentary sovereignty? Nobody knows. The result is skateboard politics: an empty thunder of promises, then reckless plunges and then crashes into failure.

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But meanwhile, and quietly, something utterly transforming has been seeping into politics. The law is beginning to infiltrate the tattered old doctrine of parliamentary sovereignty. It’s years since “judicial review” of government decisions began to wear moth-holes in the doctrine’s fabric. Now the Brexit arguments have jerked the process into high gear: the supreme court’s decision to strike down Boris Johnson’s prorogation was a blow in favour of democracy. But Gina Miller was mistaken to dash out of the courtroom and proclaim a victory for parliamentary sovereignty. In the long term, Lady Hale and her judges have torn a huge rent in that doctrine. They have brought closer the moment when all the rents and moth-holes run together and parliament (as in all modern democracies) becomes subject to supreme constitutional law.

The other day, Michael Ignatieff – once leader of Canada’s Liberals – disconcerted his British TV hosts by saying that British democracy was working well. What I think he meant was that the civilities were still preserved: “the honourable member” and so forth. Politicians see it differently, after three years of increasingly rude and aggressive language flung about the Commons. But Ignatieff, who knows England even better than his native Canada, sees further. As yet, Brexit has not led to fist-fights in the chamber, as seen in Ukraine or Italy. This is interesting, as in the last century MPs from time to time brawled round the dispatch box, even when sober. But it’s half a century ago that Bernadette Devlin sprang across the floor and belted Reggie Maudling, the home secretary, across the face. Today, passive contempt for the other lot is more common than active fear and hatred. For the moment, basic civility survives between Britons, who take their frustrations out on foreigners.

But the deepest change since 2016 is the weakening of the United Kingdom’s inner bonds. Theresa May went around preaching about “our precious, precious union”. This puzzled me, given massive English indifference. Ask somebody in Durham or Exeter why the union matters, and you get a blank stare, a shrug and perhaps a mumble. Then I understood: it wasn’t Scotland, Wales or Northern Ireland that was “precious” to her, but “the union” in the abstract – a sort of legitimising halo hovering over Westminster’s anointed. It’s a cult confined to Britain’s ruling caste and, of course, to Scottish and Irish unionists who genuinely have something to lose.

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The “great rest of England” seem to have felt for many years that if the Scots want to leave, “it seems a pity but it’s their right”. Few southerners would feel diminished. Many in England believe that England subsidises Scotland (an incorrect belief, according to SNP leaders, among others – who point to the years of Scottish oil revenues flowing to the Treasury).

Since 2016, Scotland’s heavy vote to stay in the EU, and the SNP’s incessant campaigning against any sort of Brexit, have become a severe irritant to “British” politics. Devolution is working more scratchily month by month, and the common English assumption for the past few years has been that Scottish independence is inevitable. Curiously, this is not how it looks in Scotland, where minds change slowly and where it’s far from certain that the next independence referendum will drag the yes vote over the line.

In the union of four nations, one – England – has 85% of the population. What the past three years have shown is that the big partner is no longer concerned to put its own interests behind those of the others. A poll this year showed that Tory voters would be ready to “lose Scotland” (revealing words) if that ensured Brexit. In turn, devolution only made sense when all four nations were inside the European Union. If England in 2019 can no longer remember why the union with Scotland and Northern Ireland once made sense, Brexit has delivered the United Kingdom to the hospice of history.

• Neal Ascherson is a writer, journalist and historian

• This article was amended on 29 November 2019 to attribute to the Scottish National Party an assertion that it is incorrect to say that England subsidises Scotland.