Sometimes it is the little details that tell you everything you need to know. So when, as Politics Home revealed yesterday, the chief whip meets Tory backbenchers to assuage their concerns over the transitional arrangements for the fishing industry as the UK edges its way out of the EU and tells them not to worry because, look, “It’s not like the fishermen are going to vote Labour” you know there is something deeply wrong at the heart of the government.

This, remember, is what passes for the government’s intelligence unit. And with leaders like this, who needs enemies? It is not evident whether the ignorance is more startling than the complacency or vice versa but neither appraisal reflects well on Julian Smith, the chief whip.

Spectator readers will not need reminding, even if the chef whip does, that the threat to Tory constituencies in Cornwall does not primarily come from Labour and nor does Labour challenge the Tories in fishing constituencies such as Moray and Banff and Buchan where, though this is evidently news to some senior government ministers, the SNP – that little known, rarely-in-the-news, party – were the power in the land until last June. The northeast of Scotland, indeed, was the SNP’s stronghold for more than a generation.

Still, as data points go this is a useful one, reminding us yet again that this government still does not fully appreciate that the United Kingdom has changed. What was once will not be again and many of the old rules and certainties no longer apply.

This extends rather further than a ham-fisted and ignorant chief whip. The ongoing tussle between the Westminster government and the devolved parliaments may well be, as the commentator David Torrance has winningly observed, “the world's most boring constitutional crisis” but it remains quietly important nonetheless. Making Brexit compatible with devolution is not just a matter of hamming square pegs into round holes; it slices deep into the heart of modern British constitutional reality.

Westminster remains the supreme authority as a legal and technical matter. But legal, technical, realities are not always congruent with political realities. The game has changed and it is played by new rules. Certain things which could be done in the past can no longer be done. Legislation for Scotland and Wales is a delicate matter these days. Westminster must take account of the devolved parliaments’ prerogatives.

Brexit, it is is true, complicates this. Had the UK not been a member of the EU at the time the devolution statutes were written, they might very well have been arranged differently. The devolved parliaments’ complicate life for the UK government but that is hardly their fault. They exist and have to be paid some attention.

If you were starting from scratch this is not what you might have built but then the entire British constitution is a jury-rigged contraption of Heath Robinson complexity. You wouldn’t necessarily set out to emulate much of it if you were indeed charged with starting from first principles. Nevertheless, it is what it is.

Ministers and officials in Whitehall still do not seem to understand this. The SNP government in Edinburgh has its own reasons for complicating Brexit but the notable aspect of the current constitutional impasse is the cross-party agreement in Edinburgh that devolution prerogatives must be respected. Scottish Tories can barely contain their frustration. The “dinosaurs” in Whitehall don’t understand the new realities of British constitutional life, one influential Tory insider says.

The Treasury, as so often, is considered especially culpable in these matters but it goes beyond just the Treasury. David Mundell, the secretary of state for Scotland, has spent considerable amounts of time and effort trying to persuade his cabinet colleagues to understand that a Holyrood-Westminster agreement does actually matter.

Understandably, perhaps, the UK government looks to the longer-term. It has no desire to see future trade deals complicated by extra demands made by the devolved parliaments. In agriculture, especially, it fears that the Scots and the Welsh will have their own agenda and their own set of demands that might make reaching those deals more, not less, difficult.

As indeed it would. But so what? Apart from anything else, the Scots and Welsh are right to worry that the UK government will not take account of their distinct priorities when it comes to negotiating trade deals with, say, the United States or Brazil or Australia. Agriculture is only a small part of the UK’s overall economy but (like fishing) it matters significantly more in some places than others. Some of those places now happen to have parliaments of their own.

The United Kingdom is not a unitary state. It is, informally, a federal entity now. That means old certainties must be recalibrated to take account of new realities. Happily, there is ample precedent for this. In Canada, for instance, the provincial governments have, over time, gained a de facto right to have their concerns listened to in the matter of trade deals. These have previously been signed subject to ratification by the provinces; increasingly that provincial approval is looked for before anything is agreed. That provides a template for the UK’s future too.

That is, unavoidably, messier and more complicated and more difficult than it would have been in the pre-devolution and pre-EU eras. But so be it. The Union has changed and so has the psychology of Union. As a conceptual matter, it is more provisional than it was; as a political matter it is more formal than it used to be too. Scotland and Wales have two governments, not one and that demands something more than mere consultation. It needs agreement.

That necessarily limits Westminster’s ability to do as it pleases. That should be thought a feature, not a bug. One day even the chief whip and other leading members of the government may be able to appreciate this. If Brexit complicates that new reality then so be it.