Sometimes it just takes one word to tell a whole story. It did for Alister Jack in the House of Commons this week. The Scotland Secretary was answering questions about his government’s unprecedented decision to block Holyrood’s gender self-ID bill.

Up popped Tory Laura Farris to congratulate him for, as she put it, “standing up for women’s sex-based rights”.

Then – after pausing amid audible protests from SNP members – Ms Farris asked if Mr Jack agreed “that it is essential for every corner of the United Kingdom to reach an agreed position on the age limit for a gender recognition certificate?”

His reply: “Absolutely.”

I think this monoverbal response was the most telling of the entire debate.

Why? Because it exposed one of the most fascinating but least often articulated backdrops for the entire white-hot controversy about trans rights.

And that is that there are politicians who do not think laws should vary within the UK. On anything, frankly.

READ MORE: Dead-end approach from British nationalists

This is not a column on the rights and wrongs of gender self-ID. There are plenty of those already.

No, what I want to talk about is whether MPs like Ms Farris and Mr Jack are willing to accommodate different approaches to hot-button issues on either side of the border. I don’t think they are.

And that is a position that goes way beyond self-ID, whatever you think of that issue.

Ms Farris and Mr Jack clearly believe that the age at which people can legally change their gender should be the same everywhere in Britain.

This proposition merits scrutiny. Because – rightly or wrongly – young people in Scotland are sometimes asked to take more responsibility for various aspects of their lives earlier than their counterparts in England.

For example, young people in Scotland and Wales can vote at 16, though not for Westminster. Those in England and Northern Ireland have to wait till they are 18.

HeraldScotland: Trans rights supportersTrans rights supporters (Image: free)

This, of course, is a recent development, a product of the devolution settlements of the late 1990s. But there is nothing new about legislative diversity among the nations of the UK, even on matters of family, and personal freedoms. This predates devolution and even democracy. It is a product of the Treaty of Union under which Scotland kept its own law if not its own legislature.

So, in perhaps the most notorious recent example of differing laws, homosexuality was only legalised in Scotland in early 1981 – more than 13 years after England and Wales.

Indeed, the idea that law changed at Gretna is written in to England’s literary canon. Think of all of all the young couples in Jane Austen novels threatening to elope to Scotland – where the age of marriage without parental permission was lower.

Regency authorities – when Scotland had no meaningful autonomy – did not seem to demand uniformity of law. They tolerated diversity. So why don’t modern Tories?

Well, they say they kiboshed self-ID in Scotland because it cut across UK-wide equalities legislation. Very esteemed jurists disagree on whether this stance will hold up in the Supreme Court. We will have to wait and see what judges think.

READ MORE: The unionist blindspot

There is nothing particularly unusual about varying laws in multi-polar, multi-national or multi-jurisdictional states. Even the Soviet Union struggled to impose, for example, a single agreed age limit or terms for marriage. But diversity of laws can cause legal and political headaches.

Take America, cockpit of the culture wars. Over the last couple decades the United States has got itself in to all sorts of kerfuffles over whether some white-hot issues should be decided federally, or locally.

Its Supreme Court first effectively legalised gay marriage everywhere (in 2015) but then (in 2022) decided to let states make law on abortion.

In the UK the direction of travel is clearer, towards uniformity. Which is surely an important context for the trans row?

British authorities have imposed a series of liberal reforms in stutteringly devolved Northern Ireland.

First, back in 2008, they lowered the age of consent in the province to 16, the same as everywhere else in the UK. Then in 2019 Westminster legalised same-sex marriage and abortion.

All but two SNP MPs – despite accusations of hypocrisy – backed the move to end forced birth in Northern Ireland.

A Tory backbencher picked up on this general theme in the Commons debate this week. Brexiteer Tim Loughton – citing his support for equal marriage in Northern Ireland – told Mr Jack he believed “the liberties and responsibilities of all citizens across the four nations of the UK are equal”.

Now, Mr Loughton opposes self-ID and has specific concerns about “minors” changing their gender. But he also clearly think laws on such topics should not deviate across the UK. He is not alone.

Mr Jack leans, I think, towards the instinctively British nationalist rather than the traditional unionist wing of pro-UK opinion. He has even written of his dislike for the “four nations” formula used by Mr Loughton.

I think we are entitled to ask if the Scotland Secretary supports the legal diversity brought by the Treaty of Union and reinforced by devolution.

Trans rows, of course, have their own dynamics, way beyond the British constitution. And they have cleaved divisions across political traditions, including feminism and Scottish nationalism.

Yet it is an accident of history that SNP and Tory administrations in Edinburgh and London find themselves on the opposite sides of the self-ID barricades.

Not that long ago it looked like the practice, already in place in Ireland and elsewhere, was going to become the norm in both England and Scotland.

Theresa May, like Nicola Sturgeon, was keen on making it easier for trans people to change the gender on their birth certificates.

The Tories only got cold feet on self-ID after populist Boris Johnson replaced Ms May (as it happens giving the then strikingly inexperienced Mr Jack his big cabinet break as he did so).

And thus an alliance of culture warriors and banal British nationalists was forged. Does this at least partially explain why self-ID is blocked?

In a word: absolutely.