The United Kingdom’s sense of itself and its place in the world are more in question now than they were before Donald Trump’s election. The precarious process of Brexit has destabilised the nature of 50 years-plus of UK foreign policy and international alliances.

This, combined with Trump, has now brought into the open a whole host of tensions and doubts about the so-called “special relationship” between the UK and US.

All of this should be a moment for opposition but Jeremy Corbyn’s Labour is letting the struggle with the Tories slip through its fingers. Whatever the views of Corbyn as a leader, this has and is costing the UK dear, and has long-term damaging consequences.

This offers a historic opportunity for critics of the dominant version of Britain

One of these is that the UK – as currently composed – has very little future. To add to the international and national challenges it faces is one based on the territorial dimensions of the state, the failure of the political centre to understand this, and the decline of any popular account of unionism that tells a story about the future of the UK.

Scotland, Northern Ireland and Wales are in very different places from the UK government, not just on Brexit, but where they see their future. The first two voted to remain in the EU, but it is about much more – namely, a rejection of the Britain peddled for the past 40 years based on London-centric capitalism and the global classes of the south-east.

Many in Westminster hope and pray the above just goes away, and that they can ignore it, and the rebellious Celtic nations will ultimately acquiesce in Brexit. This is the world from the Downing Street bunker and Westminster village. It just isn’t going to happen.

One clue to why is the threadbare defence of taking Scotland out of the EU. David Mundell, secretary of state for Scotland, argues that Scotland didn’t vote to remain in the EU, it voted on whether the UK should stay or go, and that is what has been decided. It is politics as pedantry and desperation.

This reveals that the rich, once all-powerful tradition of unionism in these isles is on its knees. It has been hollowed out and diminished by a host of factors – by globalisation, the decline of religion, the rise of Scottish nationalism and a renewed Scottish self-identity, and as we are currently witnessing, the emergence of an English nationalism.

Scottish nationalism has been and is a benign force: civic, progressive, pluralist, and deeply multicultural. The English nationalism we are witnessing is practically the mirror opposite – ethnic, regressive, anti-pluralist, and at war with multiculturalism and diversity.

There is something troublesome in this: of a set of competing claims, which could already be seen in Scotland’s indyref and its aftermath. This was a struggle between two nationalisms, Scottish and British – the former, out and comfortable as a nationalism; the latter, closeted and not out. This is still one of the main dynamics of Scotland and Britain, with Murdo Fraser, MSP and former deputy leader of the Scottish Tories, claiming on Twitter at the weekend that: “I dislike all nationalisms … I think it’s just daft to claim some are better than others.”

The self-denial of British nationalism has contributed to its long retreat, and challenge from an English perspective. But we should be wary for all Scottish nationalism’s moderateness of a debate restricted to the claims of nationalism. As Fintan O’Toole wrote about the experience of Ireland, nationalism is like “a rocket fuel” which can get you far in the early days, setting up an independent nation state, but it burns up quickly, and doesn’t provide a guidance for what to do once you are at your destination.

Where does that leave the UK? Scotland has already left the building. It is a different nation in its own place and already in much of how it thinks, talks and acts, quasi-independent. That isn’t going to be reversed. In my book, Scotland the Bold, I look at the consequences for Scotland and the UK and what political terrain may emerge after the exhaustion of social democracy and neoliberalism.

The “idea” of Britain is now in question and while that is a threat to the power elites and apologists for the neoliberalism and unequal Britain of recent decades, it is an opening and liberation. The transformative changes of Britain of Thatcher and Blair were brought about by the rotten, undemocratic, centralising manias of the British state and British nationalism. Their end should be no cause for mourning, but one of celebration.

This offers a historic opportunity for critics of the dominant version of Britain. Not to roll out Gordon Brown’s worn rhetoric on a constitutional convention or vague confederal Britain. But instead to imagine a Britain of different, self-governing peoples and nations, which has room for an increasingly independent Scotland, an autonomous Wales, a pan-Irish dimension, and a set of English voices and dimensions, which offers a set of collaborative relationships fit for the 21st century.

The above creates anxiety and worry in Britain’s political classes. That is a good thing. The way the UK, the British state and its elites see the world is now being brought into question by the forces of disruption and change they once claimed to represent. Status quo Britain is no longer viable. The future is going to be much more disputatious, diverse and fragmented, with multiple voices representing what used to be a more homogeneous Britain. That’s exciting and liberating. The old imperial version of Britain that continued under the Thatcher and Blair eras is finally coming to its end.