Adam Tomkins’ suggestion that the UK should morph from a consent-based union of equals into a constitutional forced marriage contains all the classic elements of modern Unionist thought.

Guaranteed to infuriate Yessers by suggesting a treaty between two independent states can be retrospectively replaced by a Hotel California-style unquittable union — check.

Guaranteed to cement the Tories’ reputation as the slightly-crazed, hardball members of the Better Together team for current electoral purposes — check.

Guaranteed to fall apart as a proposition after five minutes of serious examination — check.

But also guaranteed to momentarily deflect attention from the fact Conservatives have now abandoned any effort to defend their sacred status quo — check.

Opinion polls suggest support for independence sits at a steady 50 per cent plus — aided by Brexit and Boris Johnson’s premiership — and the SNP look likely winners of the May 6 elections. Is it any coincidence that once it becomes obvious the Conservatives have lost the argument, they boldly prepare to change the rules?

Or as you’ll never see on the side of any bus: if you can’t beat ’em, just stiff ’em.

Retrospectively and unilaterally changing the terms of the Act of Union would be quite a Rubicon for any Prime Minister to cross, even a serial ministerial code-breaker like Boris Johnson. It’s also an eloquent admission that Unionism has essentially admitted defeat north of the border and is preparing to resort to constitutional force.

Jings, was it only five years ago that a bright-eyed, bushy-tailed Prof Tomkins entered Holyrood on a Tory manifesto that promised ‘a fresh, positive drive to promote the benefits of the Union’ which would ‘not focus on the downsides of independence, but on the strength and values of the Union’?

How did that go? Evidently, not so well.

That’s why the velvet glove of gentlemanly pseudo-consent is being eased off to reveal the iron fist of compulsion. In Britain, might is right and Westminster is might personified. This — not cooperation, collaboration or consent — is the bedrock of the modern Union.

Attractive to those who get excited by the sight of Tory leaders on tanks. Attractive also to those with a penchant for embracing some aspects of foreign constitutional practice whilst cheerfully jettisoning the rest.

Let’s take America. We could be here till doomsday debating whether the Tenth Amendment to the US Constitution allows states to secede. What isn’t in doubt is that individual states signed up knowing the deal in advance. I realise retrospectively changing international agreements is quite big in Tory circles these days, but it doesn’t impress anyone else.

In 1789, only one of those US states — Vermont — was constituted as a country in its own right. In 1707, Scotland had been an internationally recognised independent country with its own parliament, education and legal systems for more than three centuries. It chose to enter a supposed Union of equals. The terms can’t just be re-written by one signatory now.

Finally, the USA has remained stable but that’s mostly because of the fair and unassailably equal distribution of powers between the federal government and member states built in from the start — not surly compulsion.

Properly federal states within the US and Canada cannot have their rights undermined at a stroke by wheezes like the Internal Market Bill or the current Westminster attempt to block Scottish legislation.

Indeed, the US simply would not exist as a union today unless the rights of individual states were absolutely guaranteed when they signed up. Not a single state would have accepted those rules being changed unilaterally by Washington, 200 years down the line.

And not a single state would ever have agreed to conveniently fuzzy assurances of autonomy like the ‘Sewel Convention,’ where Westminster agrees it will not ‘normally’ legislate in devolved areas without Holyrood’s permission.

The new arrangements steam-rollered through to facilitate Brexit have proved that such conventions aren’t worth the paper they aren’t written on. Normal is switched on and off at will by a Westminster government that makes convention up as it goes along, fails to collaborate or consign any verbal commitments to print. No state in a federal system — and no Supreme Court set up to block sharp practice — would tolerate such ad hoc arrangements for a nanosecond.

If Prof Tomkins wants to embrace one aspect of the pre-agreed, fair-minded, stability-inducing federal structures of the US and Canada, he should embrace all of them. But of course, the Conservatives don’t want a federal approach even if it includes a mafia-style, ‘no-exit’ clause because they don’t actually want to share power in a permanent, agreed, statutory way. And that’s what federalism means.

To quote Ciaran Martin, former senior civil servant and architect of the Edinburgh Agreement turned Oxford professor, federalism cannot happen in Britain without abandoning the centuries-old doctrine of parliamentary sovereignty. He concludes that will simply never happen.

As for the suggestion the UK should copy Spain’s ‘indivisible union’, really? Its 1970s constitution — outlawing ‘breakaway’ republics — was reluctantly agreed by Basques and Catalans in the wake of a bloody dictatorship. Once again, the ‘no secession’ clause was there at the outset, not tacked on later a la Tomkins. Still, it hasn’t worked. Spain is being held together by fiat, repression and imprisoning democratically elected leaders.

Is that where we are heading? Mercifully, probably not.

To quote another senior civil servant turned constitutional critic, Philip Rycroft, the instinct to preserve the Union is ‘not in the bloodstream of the UK state.’ In short, Number 10 doesn’t really care enough to produce more than a few Union Jack emblazoned spending projects for Scotland.

In all likelihood, Prof Tomkins — not standing for re-election in the Holyrood elections — is flying a final, colourful and attention-diverting constitutional kite, because if his proposal was implemented, the ‘world’s most successful union’ would be over at a stroke.

Unless that is now what the Conservatives really want?