Boris Johnson leads the Conservative and Unionist party, so named to mark its opposition to Irish Home Rule at the turn of the 20th century. If, as now looks quite possible, the union of England, Scotland, Wales and Northern Ireland is set to fracture, it will be because the prime minister's party has left behind its history. The Tories have grown indifferent to England's bonds with its Celtic neighbours.
By temperament, Johnson has never been much interested in Scotland, where Nicola Sturgeon’s Scottish National party is pressing for a referendum on independence. His attachment to Northern Ireland is weaker still. After promising the Tories’ allies in the Democratic Unionist party that he would never allow an economic border in the Irish Sea, he signed up to a Brexit deal with Brussels that does precisely that.
The campaign to leave the EU led by Johnson in 2016 was in essence an expression of English nationalism. He opposes separation with Scotland only because it would deliver a grievous blow to England's international stature. The break-up of a union spanning more than three centuries would be quite likely to destabilise the UK monarchy. More important for Johnson, it would probably see him turned out of office.