The British have long had a penchant for chasing the quick money. This ambition drove our empire building and then, when Europe looked the place where growth and easy pickings lay, drove us into the Common Market. Now the European Union doesn’t look so great, we are turning our eyes elsewhere – we want some of that American and Asian action.

Seen through this lens, Brexit is just another twist in perfidious Albion’s long history of opportunism – from Drake’s raids on Spanish treasure ships, to looting India, then to embracing the European project (with the opt-outs from things we didn’t like) before leaving.

Now our aim is to build whatever trade partnership with the EU we can without the EU’s irritating constraints, but leaving us free to cut deals with any fast-growing country or region in the world we choose, while controlling immigration. Open to the globe, but with as few migrants as possible. Sounds good? It is nonsense.

For a start, our ancestors were more clear eyed than today’s Brexiters. They had a consistent line of sight on what was possible – and impossible. As today’s historians write, the British made up empire as we went along. We cut deals with chieftains and rulers where they were obviously too powerful to be conquered; we only colonised where it was comparatively easy and the short-term profits in creating sugar, cotton and tobacco plantations fabulously high. It was consistently pragmatic work in progress and not ideologically driven.

Yet misguided rightwing ideology now governs our affairs. For the British Eurosceptics, Europe has become so toxic nobody can think straight. Thus a footnote to the five-paragraph Brexit bill declares that Britain is to leave Euratom, the organisation set up in parallel with the EU to ensure safety standards in the European nuclear industry. It has the power to cut deals with countries outside the EU and is one of the linchpins of the global nuclear safety order. But none of that now matters. Britain is to compromise its nuclear safety standards, lose the world-leading research facilities in Culham and imperil an industry in which it is comparatively strong – for what? Nothing, except to appease the rightwing press, Ukip and Tory Eurosceptics.

The same ideologically driven delusions are playing out across the board. The core assumption of the Outers is that the world of the next 30 years will be like the world of the last 30 – of growing multilateral trade and effective supranational institutions such as the World Trade Organisation to underwrite and police it. Here, the new Britain, free from the “dead hand” of Europe, will hunt for the global growth hotspots as it chooses. Except there are two giant roadblocks – and one lie. The first roadblock is that this is not the world the US under Donald Trump wants to sustain; it has created “carnage” in the US industrial heartlands, to which trade protection is the appropriate response.

The second is that even if the liberal trade order does pull through, however battered, Britain faces the impossible task of compensating for what the National Institute of Economic and Social Research calculates will be a 22% drop in Britain’s exports to Europe. Mathematically, even if the rest of the world signs instantaneous trade deals with us, this cannot be achieved. This is not least because the areas in which we are most competitive, such as finance, are areas where we do well in the EU but typically are not covered by trade deals in goods. No country, unless mad, would put itself in this position.

But we are deranged by the lie that the EU prevented Britain from being a global actor in trade. Wrong. We were free to sell anywhere we liked under WTO rules, just as Germany and Holland do. What is happening is folly. Theresa May’s trip to the US was therefore not just about appearances; it was trying to cap the risks to globalisation, multilateralism and pivotal supranational institutions posed by Trump. If Britain locks herself out of Europe only to confront a world of stagnating trade, dog-eat-dog bilateral deals and the collapse of the great UN institutions, of which the IMF, WTO and World Bank are all part, then the British economy faces years of stagnation and decline. Trump may have celebrated the special relationship along with the promise of a trade deal – and affirmed support for Nato – but beyond that May got nothing.

For Trump likes bilateral trade deals in which the US will scalp its putative trade partner; it conforms with his protectionist world view. Whatever May’s appeals to shared American and British values underpinning the global international order, Trump is having none of it. He did not promise to uphold the WTO or even the UN. He remains wedded to America First. May gave it her best shot, but she is about to lead Britain into economic hara-kiri.

Contrast her supplicant comments with the powerful speech by China’s president, Xi Jinping defending globalisation based on first principles – not on an elusive “special relationship” – at the World Economic Forum. “Pursuing protectionism is like locking oneself in a dark room,” thundered Xi. “No one will emerge as a winner in a trade war.” The global market, he argued, was an inescapable “vast ocean” that had to be embraced to win growth not only for oneself – but for everyone. “Any attempt to cut off the flow of capital, technologies, products, industries and people between economies, and channel the waters in the ocean back into isolated lakes and creeks is simply not possible. Indeed, it runs counter to the historical trend.”

China, declared Xi, wanted to uphold, strengthen and, more ominously, reform the institutions that governed globalisation so that voices such as China’s were better represented. Innovation, multilateralism, standing by treaty commitments and social fairness had to be the watchwords to make globalisation work better.

Amen to that, but who is to trust a deeply mercantile one-party state to deliver on such promises? How will a weakened Britain, desperate for any trade crumb, fare in China’s reordering of the global system as the US abandons it? Our empire-building ancestors, clear sightedly reading the runes, would have made common cause with fellow Europeans to keep the world open on terms that work for us, averting an “eclipse of the west”.

But we are governed by fools and knaves. This is how countries get locked in irreversible decline – when a conjunction of domestic interests and backward-looking forces makes it impossible to do the right thing. May was right. We need the world order to hold. It’s why Britain, even now, should not be contemplating a hard or “clean” Brexit – and why every MP of whatever party should resist the drive for self-destruction.