“Boris Johnson’s breath-taking reversal of his previous position on the idea of a Northern Ireland-only backstop makes October 10th, 2019 – the day of his meeting with Leo Varadkar in Wirral – a watershed in British and Irish history.” Photograph: PA

A border in the Irish sea has long been a red-line for the DUP but it seems to be one crossed in the Brexit agreement between Boris Johnson's government and the EU.

If you pass through Heathrow or any other large British airport, you will be assailed by large photographic posters declaring British greatness: Innovation is GREAT; Sport is GREAT; Design is GREAT, and so on. The tagline, of course, is Britain – all of these wonderful things are GREAT Britain.

Except that the whole concept is ruined by an awkward interloper. The copywriters had to add “& Northern Ireland” and that turbulent province wrecks the pun and deflates the slogan. “Music is GREAT Britain & Northern Ireland” doesn’t work. It creates a decidedly downbeat anticlimax.

And the ad agencies seem to be sneakily avoiding it. Last week in Heathrow I noticed three posters without the dangling appendage: Countryside is GREAT Britain; Culture is GREAT Britain; Heritage is GREAT Britain. No “& Northern Ireland”.

It is easy to sympathise – while it seems a bit harsh to imply that the bucolic splendours of Antrim and Tyrone are not so great, culture and heritage do become especially touchy subjects when you sail west across the Irish Sea.

Johnson decided, in effect, that Northern Ireland can be detached from the British train and placed on a whole other track The awkwardness and uncertainty of these posters makes them inadvertently telling images of a much larger ambivalence. The idea of Great Britain itself is under severe strain as Scotland seems destined to move towards another referendum on independence. The notion of British greatness has been weaponised politically – being great makes it unnatural and intolerable for Britain to be just another member of the European Union.

But most dramatically, that tenuous “& Northern Ireland” has never been so uncomfortable an adjunct to Britishness. It is not just the advertising copywriters who feel inclined to drop it altogether.

Line crossed Whatever turns the Brexit saga takes – and his week’s events at Westminster promise many more to come – it is clear that what Arlene Foster called a “blood-red line” has been crossed. Boris Johnson’s breath-taking reversal of his previous position on the idea of a Northern Ireland-only backstop makes October 10th, 2019 – the day of his meeting with Leo Varadkar in Wirral – a watershed in British and Irish history.

On that day, Johnson decided, in effect, that Northern Ireland can be detached from the British train and placed on a whole other track. The implications of this for the future of both islands are profound.

The scale of the volte face is clear if we remember that Johnson, as prime minister, had vehemently rejected on unionist grounds even Theresa May’s deal with the EU that involved a much less radical degree of separation between Northern Ireland and Britain. On August 19th, he wrote to Donald Tusk denouncing what was then the backstop as “inconsistent with the sovereignty of the UK as a State” because “it places a substantial regulatory border . . . between Northern Ireland and Great Britain.”

But now he has actually agreed to a vastly more intrusive regulatory and customs border in the Irish Sea. If May’s deal undermined the sovereignty of the UK as a single entity, Johnson’s blew it apart.

Mental separation This is a bell that cannot be unrung. Whether or not the arrangements for a unique form of Brexit for Northern Ireland actually come into being (indeed, whether or not Brexit actually happens) a huge act of mental separation has occurred.

In fact, we are witnessing the birth of a new country called “GB for now and NI over there”.

Just five years ago, in any discussion of the future of the UK as an entity, the factors in play were Scottish nationalism, Irish nationalism and, to a lesser extent, Welsh nationalism The shock is all the greater because the jettisoning of Northern Ireland is so casual. It is not just that supposedly hardline unionist Tory MPs were so upfront in saying that, much as they love the union, England comes first and the DUP must, as arch-Brexiteer Steve Baker put it, simply “choke down” its betrayal – a slightly nicer formulation than “suck up”.

It is that even at the highest levels of the British government, there was little thought given to what a border in the Irish Sea would mean. There was a telling moment in the House of Lords on Monday when the Brexit secretary Stephen Barclay initially claimed that Northern Ireland businesses would not have to fill out customs declarations to send goods to the rest of the UK before, after correction from the Treasury, acknowledging that “declarations will be required”. Johnson subsequently struggled to explain this in the House of Commons.

That something so basic could be so unclear tells us that we are experiencing a strange mix of the momentous and the thoughtless. A fundamental change in the nature of the UK (and thus of Ireland) is being unleashed, not through a process of careful deliberation, but in a spirit of pure reckless opportunism. Whatever gets Brexit through the night is alright. The consequences are for later.

The significance of this shift is hard to grasp because it is not part of the story most of us in Ireland recognise. Just five years ago, in any discussion of the future of the UK as an entity, the factors in play were Scottish nationalism, Irish nationalism and, to a lesser extent, Welsh nationalism. What was not being discussed was the idea that English nationalism might be the real destructive force. Yet its presence was not secret.

In 2012, four years before the Brexit referendum, the Institute for Public Policy Research (IPPR) published a very powerful report, based on detailed survey data, that warned that “an emerging English political identity may over time come to challenge the institutions and practices of the UK more profoundly than anything happening in the so-called Celtic fringe”.

One of the many staggering aspects of the Brexit saga is the decision of the DUP to ally itself to an English nationalist project that posed such a clear threat to British identity The 2011 UK census showed that, even given the option of ticking more than one box (English and British for example), a majority of people in England chose to identify as English only. Just 29 per cent of English people claimed to feel any sense of British national identity. And what is even more striking is that, in the years before the referendum, survey data were showing a very strong correlation of English feelings about what IPPR called “the two unions”, the UK and the EU – people who identified strongly as English rather than British were also anti-EU.

The institute warned that “any decision to ignore English discontentment for fear of guilt by association with right-wing populism is only likely to further feed such discontentment – and perhaps encourage it to develop more toxic undertones”.

The warning was, of course, itself ignored. And nowhere more so than within Irish unionism. One of the many staggering aspects of the Brexit saga is the decision of the DUP to ally itself to an English nationalist project that posed such a clear threat to a pan-UK British identity.

All the evidence both before and after the 2016 referendum was that the voters in England who identify as British are, by and large, Remainers. People who support one union (the UK) are very likely to also support the other (the EU).

Conversely, Leave voters, whatever their political representatives say, tend to care very little about the preservation of the UK as an entity. For them, Brexitness is far more important than Britishness: every poll has shown that Leave voters and Tory party members find the breakup of the UK an acceptable price to be paid for Brexit.

This was confirmed on Monday, in a new poll and focus-group study of English participants released by the Tory donor Lord Ashcroft. The threat that a border in the Irish Sea might pose to Northern Ireland’s place in the UK is not a big deal for most of them: just over a quarter of English respondents, and just a third of Conservative Leave voters, agree that it would be “unacceptable” for Northern Ireland to “have different laws and regulations from the rest of the UK after Brexit”.

Perhaps even more alarming for unionists is the general air of English indifference. The focus group findings are that: “Apart from the observation that ‘the religious element is very strong’, very few had any grasp of the dynamics of Northern Irish politics, which seem complicated and even mysterious to many people. Some were not even aware that Northern Ireland’s long-term place in the union was even an issue.”

When asked whether Brexit makes Irish unification more likely, the most popular response in the poll was “don’t know”. When asked directly whether Northern Ireland should remain part of the UK, just 35 per cent said yes, 13 per cent said no and a remarkable 43 per cent said: “I don’t have a view.” And when those in this latter category were asked how they would feel personally if Northern Ireland were to leave, 59 per cent said “I wouldn’t mind either way” and a further 8 per cent chose “happy to see them go”.

What has occluded this obvious reality is political piety. English nationalism is odd in that in has no real political voice. It remains taboo for any mainstream English politician – let alone a prime minister – to say that the people of England “don’t mind either way” about that complicated and mysterious place called Northern Ireland.

But sooner or later politics catches up with popular feeling. And this is what has now happened. The Brexiteers have now caught up with their base and made explicit what has always been implicit in the English nationalist project: Northern Ireland is neither here nor there.

The Brexit project may ultimately be about the English exiting their “two unions”, the UK as well as the EU. But in neither of these exits is the desired destination known Hence the weird mixture of opportunism and inevitability. Johnson seized an opportunity to keep his show on the road by cutting the Irish rope that tethers the balloon of post-Brexit Britain to the EU. But he could do so because he knows very well that his party and his potential voters have ceased to regard Northern Ireland as an integral part of their country. Once they decided that their country was England, there was already a psychological parting.

Chaotic consequences This rise of English nationalism was bound to have long-term consequences – but Brexit has made them much more rapid and chaotic. The chaos comes from the reality that the separation is not active, not planned or thought through. It is not a decision to seek a divorce. It is just a relationship that, on the English side, has gone cold and lapsed into an unresponsive apathy. That is, for Northern Ireland, even more dangerous than an upfront decision to eject it from the union. The shrug of the shoulders is less engaged than the shove in the back, which at least has to be hands-on.

The great problem for Ireland, though, is that this is not our story. In the Irish story, a united Ireland might come about as a result of a long and careful process of reconciliation on the island. But what we are actually seeing is a whole other movie, made and screened in England. It is an avant-garde extravaganza that does not bother with a coherent narrative.

The Brexit project may ultimately be about the English exiting their “two unions”, the UK as well as the EU. But in neither of these exits is the desired destination known. Something is being left but it is not at all clear what will be left behind.

All we can really say for sure is that if Brexit tells us anything it is that if you ignore deep changes in national and political identity, they will probably turn toxic. That has happened with English nationalism – the warning signs were disregarded.

We are now witnessing a sudden and jerky acceleration in what was meant to be a slow and careful reshaping of the political architecture of these islands. We had better recognise it and try to deal with it before the poison of betrayal and abandonment enters the groundwater.