I hope Scotland and Nicola Sturgeon realise how much they may yet owe Boris Johnson. If I were a Scot, I would vote for independence tomorrow. I would want nothing more to do with the shambles of today’s Westminster parliament, which goes on holiday for a month during the worst political crisis in a generation. Labour’s John McDonnell is entirely correct to reassure the Scots of their right to secede from the United Kingdom. The supreme civil right is that to self-government, and the inferior tier of a federation is entitled to claim it, not the superior one to permit.

Scotland has now voted itself a separatist Scottish National party local government unchallenged for 12 years. The party is 20 points ahead in the polls, while support for independence has topped 52%, the same percentage that voted for Brexit across the UK in 2016.

Johnson’s sidekick Dominic Cummings this week warned politicians that they “don’t get to choose which votes they respect”. That is exactly what Cummings and Johnson are doing. They are choosing to ignore the Brexit referendum pledge of frictionless trade, and Johnson is refusing to allow Sturgeon a referendum on independence. Sauce for the Brexit goose is sauce for the tartan gander. No wonder Johnson was about as welcome in Edinburgh last week as Donald Trump in El Paso.

This crisis is Sturgeon’s opportunity. Not much is certain at Westminster these days. No-dealers are fighting with no-confidencers. People’s voters are fighting with proroguers. Jeremy Corbyn is fighting with “national unity” rivals for a taxi to the palace. It is chaos.

London now faces precisely the dilemma that faced Gladstone: negotiate 'independence-lite', or face a further break-up of the UK, as in 1922

Only one thing seems sure. In the coming months, either before or after another election, the current bloc of 35 SNP MPs will be able to hold Westminster to ransom in a hung parliament, with or without the Liberal Democrats. It was mere prudence for McDonnell to suggest that Labour “would not use parliamentary devices” to block Sturgeon’s independence referendum, which she has demanded within two years, come what may.

Johnson is in a long line of Westminster leaders determined to infuriate the Scots – as a century ago they once infuriated the Irish. With the exception of Tony Blair’s partial devolution, London has simply ignored the progressive disintegration of the “first British empire”, the one that has embraced the British Isles since the Norman conquest and was cohered as a supposed United Kingdom in 1801. While France, Germany and Italy (if not Spain) have steadily assimilated their disparate provinces over time, the United Kingdom has done the opposite. Through persistent, bumbling misrule it has alienated the so-called Celtic fringe, and fuelled the fires of separatism.

Nonetheless, parliament has been the cockpit of this alienation. Ireland’s many revolts came to a head in the 1880s, in a parliamentary pact between Gladstone and Parnell for home rule in Dublin. Gladstone’s Government of Ireland bill was passed by the Commons but rejected by the Lords in 1893.

The ensuing bloodshed vitiated what might have been “independence-lite” and led to total independence in 1922. What had been almost a third of the population of the new kingdom vanished, leaving London with the political conundrum of Ulster to the present day.

The misgovernment of Ireland was then replicated in Scotland, culminating in Margaret Thatcher’s pilot poll tax in 1989. Scottish separatism, previously unthinkable, surged overnight. By the 2015 election, Scottish nationalists had won 56 out of Scotland’s 59 Westminster seats. Poll tax abolition and devolution had not worked.

London now faces precisely the dilemma that faced Gladstone: whether to negotiate “independence-lite”, or face a further breakup of the UK, as in 1922. Central to this is Brexit, and what it does to the Scottish independence cause.

Referendum voters asked to decide on their nation’s destiny rarely look to personal gain or commercial advantage. England’s Brexit voters were concentrated in parts of Britain with most to lose from Brexit. Economists may assert that big countries enjoy economies of scale. The Scots may be told time and again by England that they are better off under London and its subsidies. But voters are more concerned with national identity, pride, self-reliance and local accountability.

Scotland has the same population as Denmark, with much the same resources, infrastructure and talent. Once it was richer by far. Today its GDP per person is £33,000 against Denmark’s $63,000 (£52,000). I am sure the slide lies in Scotland’s long economic dependency on the UK.

Denmark would no more think of re-entering its former union with Norway than of leaving the EU. It enjoys small-state autonomy, and clearly prospers from it. It is a rich, happy country.

Sooner or later, London will be forced to grow up and recognise that it has sacrificed the right to rule the British Isles. Ireland has gone and Scotland will clearly go one day. Whitehall should take the initiative and prepare a fiscal and legislative independence package; one that withdraws Scots MPs from Westminster and sees Scotland rejoin the EU, but keeps travel, currency and citizenship ties in place.

If Johnson really does stick to no deal – or its bastard sibling, hard Brexit – he will drive Scotland to separation. Like it or not, this means borders. To the absurdity of inspection posts in Fermanagh and Tyrone will be added posts in Berwick and Carlisle. Johnson will then have to tell the Queen that she must be just Queen of England – and perhaps Wales.

• Simon Jenkins is a Guardian columnist

• This article was amended on 9 August 2019. An earlier version gave Scotland’s GDP per person as $33,000. That should have been £33,000.