After the election of Donald Trump in 2016, an American friend compared the nativist populism of the United States with the state of Brexit Britain. “You think it’s bad that Britain voted to leave the EU,” he told me. “America has voted to leave itself.”

Four years later, things look a little different. Having indeed taken leave of its senses, America has now been rescued – not for the first time – by its citizens of colour. Polling data shows that without minority-ethnic voters, many of whom had to overcome deliberate and systemic attempts to suppress their participation – the nation’s constitutional and political integrity would have endured a further four years of Trump’s wrecking ball.

Under cover of the past four years of regression, the British government has been running riot. However badly our leaders behaved, though, they knew there was a larger, more powerful democracy behaving even worse. Conservative attacks on the independence of the judiciary, for example, may represent an unprecedented assault on our constitution. But for Trump, lashing out personally at individual judges on Twitter became routine.

The British government’s relaxed attitude about violating international law has prompted the condemnation of nearly all living former prime ministers. But Trump led the way in tearing up international agreements and withdrawing from multilateral organisations.

And then there is race. In Britain we have had to endure an equalities minister who suggests anti-racism reading materials are illegal in school, a foreign minister who derided Black Lives Matter as a Game of Thrones spoof, and Boris Johnson himself, as ready to insult black children in Africa as he was the black president in the White House. Vice-president-elect Kamala Harris is said to “hate” Johnson for claiming Obama held a grudge against Britain because of his “part-Kenyan” heritage. The prime minister’s comments have not aged well.

The Kenya reference was not accidental. Much of Johnson’s political strategy rests on foundations of imperial pride and colonial nostalgia. That was compatible with the “special relationship” when the American president was, like him, similarly smitten by an imagined great white past. Lamenting the decline of this relationship has become a national pastime in Britain – traditionally at just such moments as this, when a change of guard in the White House threatens the status quo. What is clear is that, insofar as the special relationship does exist, it’s rooted in “shared cultural values”. This phrase, whenever deployed by Britain, is almost always code for: “We colonised you once, and how well you’ve done from it.”

'She's made us proud': Kamala Harris's ancestral village celebrates election win – video

But empires, inconveniently, have a habit of striking back. And so the victims of British colonial abuse in Ireland have, through a twist of fate, lent their ancestral memory to the new US president. When Joe Biden visited County Mayo in 2016, he heard how his home town experienced the worst of the potato famine – even by the catastrophic standards of the nation as a whole – the entire population “gone to workhouse, to England, to the grave”.

Kamala Harris’s heritage gives her more in common with many British people than it does with most Americans. Her grandfather worked for the British colonial government in India, where he strived for independence from the white supremacist ideology of the British empire. The power behind this empire earlier pioneered the enslavement of Africans that led Harris’s father, Donald Harris, to be born in Jamaica.

Tories pumped with pride from this same history – gloriously bragging in song that “Britons never shall be slaves” – are unlikely to find its seductive power holds much sway within the incoming US administration. The government ignored British ethnic minorities when we offered the truth of our own lineages to counter this propaganda. Ignoring the president and vice-president of America is slightly harder to pull off.

That leaves Johnson looking particularly fragile and exposed. This week one of his predecessors, John Major – no stranger to strained relations with America when he was in office – warned that “complacency and nostalgia are the route to national decline”. Britain needed a reality check, Major cautioned. “We are no longer an irreplaceable bridge between Europe and America. We are now less relevant to them both.”

Much of Britain’s decline is structural, set in motion long before Johnson took office. But if you wanted to exacerbate it, you’d struggle to find a more effective path than the one we are currently on. We have never in modern times endured anything quite as extreme as the toxic assault on America’s political culture left behind by Donald Trump. As usual, ours is a poor imitation. And like all cheap fakes, it’s not built to last.

Afua Hirsch is a Guardian columnist