Who are you and what have you done to my friend Britain? This is what the country’s one-time admirers from abroad — I am one — are asking as we watch it descend into increasingly vituperative politics and a policy course akin to self-sabotage.

The UK, after all, has always seemed a sensible country more interested in practical questions than grand visions. Against overblown rhetoric on the continent, it offered plain speaking. Its establishment could consistently be counted on to disarm ideological flights of fancy — whether the totalitarianism of right and left or schemes for European federalism — with a bemusement that was equal parts dull and droll.

In short, by its temperament and its political practice, Britain provided temperance in a part of the world too prone to extremes. So what happened? If that temperance has now evaporated, we who count ourselves friends of Britain (as we thought we knew it) should ask whether we were mistaken all along.

It was an indisputable moment of honour for Britain in the 1930s to resist the extreme ideologies that took over most of Europe, in defence of a moderate liberalism. But in our gratitude for this bastion of moderation, perhaps we mistook an exceptional performance for a permanent national character.

Take a closer look, and Britain is a country as much defined by extremes as by their absence. This is true of its history. The English nation chopped off the head of its king more than a century before France did. And Britain was no laggard behind European peers in the cruelty it applied to its colonial projects.

In the second half of the 20th century, the UK arguably delivered the biggest swing of the pendulum (with Sweden) from the most thoroughly socialist economic model in the postwar west to the most radical rollback of the state.

Immigration, too, is a story of one extreme replacing another. Britain was long a safe harbour for people fleeing persecution and poverty, the destination for the Kindertransport and the Habana’s 4,000 Basque refugee children. Even the abominable treatment of the Windrush generation in recent years paradoxically reflects how open the UK once was to people from elsewhere in the Commonwealth. It is because so little was demanded in the way of paperwork to settle in Britain that the absence of documentation could later be used against immigrants.

It is not just changes over time. Britain is riven by contemporaneous extremes, too. It has, in London, Europe’s only true global city; it also has badly neglected left-behind towns and areas cut off from a functioning economy. Its universities are world-class, but it has the worst rates of literacy and numeracy of any large western European country. It marries an inordinately lucrative global financial sector with a subpar economy run on low-wage, low-productivity labour rather than productivity-enhancing capital investment. Then there is the class system and the still-unsettled project of a multinational state.

The inequality between the UK’s different geographic parts is Europe’s most extreme when measured by purchasing power-adjusted regional gross domestic product. West Londoners’ real GDP per capita is more than six times the EU average. Greater London (commuting may distort the central city’s figures) is one of Europe’s most prosperous agglomerations. At the same time, the worst-off British regions have per capita real GDP levels similar to the poorest areas of Portugal and Spain.

Those are a lot of extremes to pack into a middling country that has lost its way. To borrow a Marxian term, the social contradictions are more acute than elsewhere and may have been so more often than not throughout history.

This make sense of the violent swings in national direction in the past, and again in 2016. The more tense are the pent-up springs of opposite extremes forced together, the more disruptive is the snap when it ultimately comes.

If this is true, there are three lessons to draw. The first is that the Brexit mess was unavoidable. In a country of such extreme opposites, any big change of course must involve many painful disruptions. The groans you can hear from Westminster are the sound of the belated realisation that Brexit will be an exercise in delivering disappointments.

The second lesson is that the UK’s future is now deeply unpredictable. Bringing to the surface the repressed tension of British society leaves deep uncertainty about how those contradictions are ultimately resolved. That is the thing with opposed extremes: their force may give the semblance of stability to the revolution simmering underneath. So no one should be confident in predicting where the process set off in June 2016 will take the country a year, a decade or a generation hence.

The third lesson is for outsiders who had become used to seeing Britain as the ballast of good sense in an impetuous Europe and the world at large. We should mourn the fact that we no longer can. When a friend has changed beyond recognition, it feels like bereavement; when it turns out we were misled all along, it feels like deception.

It is reckless to expect the UK’s turmoil to be shortlived. Its partners, in Europe above all, must hope for the best but plan for the worst. We will only know again what sort of country Britain is when our UK friends have agreed what they want it to be. We wish them luck; they are going to need it.

martin.sandbu@ft.com

Letters in response to this article:

EU membership has contributed to divisions / From Gordon Bonnyman, Frant, E Sussex, UK

How has Britain found itself in this dead end? / From Hugh Malim, Milan, Italy

Yes, there will be problems, but the democratic decision must be respected / From William Shawcross, St Mawes, Cornwall, UK

Copyright The Financial Times Limited . All rights reserved. Please don't copy articles from FT.com and redistribute by email or post to the web.