The nuclear option of the Isle of Man breaking away from the United Kingdom began to look a little less irrational this month.

’Mexit’ was moved off the bottom of the agenda by the neo-colonialist behaviour of politicians at Westminster.

Supposedly progressive MPs came close to imposing legislation on the Crown Dependencies in disregard of their special constitutional status.

Crisis was averted when the UK Government withdrew the relevant item. But an apparent majority of the House was prepared to force the islands to make public their registers recording the beneficial or ultimate ownership of offshore companies.

The move followed the imposition of public register legislation on the Overseas Territories, reflecting a belief that the UK should dictate the standard in financial regulation for the whole ’British Family’.

That seems a strangely Victorian and paternalistic position for left-wing MPs to adopt.

Complete independence for the Isle of Man has never appeared a realistic prospect except in the eyes of Manx nationalists on the fringes of local politics.

But in a world turned upside down by Brexit and Trump, things previously unthinkable have become conceivable. Is ’Mexit’ still a mad idea or should we start to take it more seriously?

And if independence is the solution, what is the problem?

The elephant in the room is the nebulous nature of the island’s unwritten relationship with the UK. It has never been comprehensively defined, nor has it ever been fully tested.

A constitutional grey area, it is open to interpretation and has the potential to produce interventions that are unacceptable to insular notions of autonomy.

There is broad agreement, though, on the general description of the relationship. The Isle of Man is internally self-governing, so is autonomous in domestic affairs, while the UK is responsible for the island’s international relations, defence and good government.

Drawing the precise boundary between the domestic and the international is where the difficulty can lie.

In modern times there has only been one instance, in the 1960s, of the UK legislating for the island without its consent.

After the UK blocked moves for the newly established Manx Radio to broadcast internationally, Tynwald refused to co-operate in outlawing pirate radio stations, one of which was Radio Caroline, anchored in Ramsey Bay.

The result of the stand-off was the extension to the island of the UK Marine etc Broadcasting (Offences) Act 1967, justified by the UK on the grounds that it was under an international obligation to take action against unauthorised broadcasters.

The episode made it clear that in the last resort the UK could impose legislation where this was deemed necessary to ensure compliance with international law.

This understanding was reinforced by two subsequent controversies when the UK came close to intervening, over judicial corporal punishment (the birch) in the 1970s and gay sex in the 1980s. In both cases Tynwald was reluctant to accept rulings of the European Court of Human Rights requiring Manx law to be changed.

Meanwhile, the Kilbrandon Report of 1973 became the nearest thing to an authoritative text on the relationship between the UK and the Crown Dependencies.

Established by the Wilson government in response to calls for home rule for Scotland and Wales, the Royal Commission on the Constitution completed its work under the chairmanship of the Scottish judge Lord Kilbrandon.

The report observed that while the UK parliament’s power to legislate was unlimited, it was the practice not to legislate without the consent of the dependencies ’on matters which are of purely domestic concern to them’.

Strict adherence to this practice had established a constitutional convention ’whereby Parliament does not legislate for the islands without their consent on domestic matters’.

Sounds sensible, but when exactly does a domestic matter become an international one? This is the million dollar question that has never been conclusively answered. It is likely that if tested the UK and Crown Dependencies would have differing views on this critical point.

In the Isle of Man, at least, our experience of actual and potential UK interventions on international matters might suggest that its power under this heading is limited to issues of applicable international law. But this has not been confirmed.

The lack of clarity in the relationship was highlighted most recently by the UK parliament’s Justice Committee, reporting in 2010.

The committee quoted the comment of the then Justice Secretary Jack Straw that the relationship with the dependencies was a ’subtle’ one. On the UK’s responsibilities it concluded ’the precise extent, and limitations, of these responsibilities are unclear’, noting that the limits ’have never been tested’.

Responding to the committee report on behalf of the HM Government, the Ministry of Justice took a noticeably broad approach to the UK’s responsibilities and powers.

The MoJ stated: ’The United Kingdom respects each Crown Dependency’s laws and policies as the expression of the will of a democratic government with the power of self-determination.

’The UK government is responsible for the Crown Dependences’ international relations and ultimate good governance and has the commensurate power to ensure these obligations are met.

’Whilst the UK does not require the Crown Dependencies’ policies to closely mirror those of the UK, the UK will look to resolve anything which appears to be fundamentally contrary to current UK principles or interests with the Crown Dependency concerned.’

The MoJ did agree with the Justice Committee’s assessment that the power of intervention in the name of good government could be used ’only in the event of a fundamental breakdown in public order or of the rule of law, endemic corruption in the government or the judiciary or other extreme circumstance.’

The Crown Dependencies would probably concur with the UK on this aspect of the relationship. But on others there is scope for divergence, particularly over the full implications of ’international responsibility’.

In the meantime the constitutional integrity of the islands could be fatally undermined if MPs were seen to impose measures upon them on no better basis than metropolitan political fashion.

’Mexit’ still seems a pretty bonkers scenario. How could it not when we see the absolute chaos caused by idealistic nationalism across the water?

Let’s hope the Isle of Man is not pushed towards the cliff edge of independence by the new imperialism of the politically correct.