Scotland’s first minister, Nicola Sturgeon, yesterday reiterated her promise to give her country another referendum on independence within the next two years. After all, if there’s an argument for Britons having another Brexit referendum just three years after the last, Scotland can surely have another on independence after five years. Besides, Sturgeon must be thinking, was there ever a better moment for divorce from a marriage so dysfunctional as the United Kingdom?

Some might wonder what new horror history could unleash on poor Britain. After Brexit comes Scexit. But Sturgeon is in the ascendant. The weekend’s polls put her SNP’s lead over all other parties at between 23 and 26 points, astonishing for a governing party that has been in office for 12 years, with Sturgeon as its leader for five. In that time, the popularity of Scottish independence has risen to an unprecedented 49% support. With an electorate that strongly rejected leaving the EU, Sturgeon reasonably calculates that this is the moment to strike. The difficulties of the Northern Ireland backstop will pale into insignificance beside an Edinburgh one.

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Scottish nationalism is not going to die. Sooner or later, Scotland will get an independence of some sort, if only because devolution/partition is the direction of travel in an over-centralised Europe. In every country – Germany, Italy, France, Britain and, this weekend, Spain – the great postwar political families are disintegrating. Party blocs no longer correspond to class or economic interest. Minority loyalties are re-emerging, mostly at the extremes, shattering the coherence and discipline of party governments. The result everywhere is to require negotiated coalitions, and these tend to grant disproportionate power to fractious provinces, witness Northern Ireland and the Catalans.

The fate of the United Kingdom, and not Brexit, is the cardinal issue of current British politics. Sturgeon is about to present London with a direct challenge: permit another legal referendum or we will find ways of staging one ourselves. During that of 2014, David Cameron bribed the Scots with more devolution and a safeguarded exchequer subsidy under the Barnett formula. It worked. Sturgeon would vastly enhance her case – not least with wider British opinion – if she renounced the subsidy, and talked the details of an oil, taxation and customs union deal with a post-Brexit London.

The concept of independence is now as entrenched in Anglo-Scottish relations as it was in Anglo-Irish ones before 1921. But as with Brexit, independent does not mean “independent”. It means shades of good neighbourliness, as has been forged with Ireland. The validity, let alone the stability, of Britain’s Celtic empire has never been resolved. It would stand to Theresa May’s credit if she now gave “independence-lite” the same attention she is being forced to give Brexit-soft.

• Simon Jenkins is a Guardian columnist