THERESA May’s speech to the Scottish Conservative Conference in Glasgow at the weekend turned out to be, in its way, a masterpiece of omission and ambiguity. However, a careful reading can reveal how menacing it really was.

Amid a lot of Tory flannel, the key passage concerned what the Prime Minister called collective responsibility: “We need to build a new collective responsibility across the United Kingdom, which unites all layers of government, to work positively together to improve the lives of everyone in our country.”

That at first sight looked like platitude, but I think it was the passage where May most meant what she said: “While fully respecting, and indeed strengthening, the devolution settlements and the devolved administrations across the UK, we [the Government in London] must unashamedly assert this fundamental responsibility on our part.”

The first thing she went on to mention was her proposed industrial strategy, “a new approach to government, stepping up to a new, active role that backs business and ensures people in all parts of the UK share in the benefits of economic success”.

Actually it’s an old approach – what we used to call, in the days of Harold Wilson and Edward Heath, the policy of “picking winners”. For example, there was the hovercraft, a particular favourite of Tony Benn, as I recall. Some hovercraft even hovered in Scotland, over the estuary of the River Dee and then between Glasgow and Largs or Rothesay. But it remained an eccentric English invention, the product of a professorial brainstorm.

Today, the world’s last example hovers harmlessly across the Solent to the Isle of Wight. It reminds us governments are no good at picking winners.

But, since May’s speech, attention has focused on what she had to say not so much about industry as about agriculture, and Scottish agriculture in particular. This was an area of policy entrusted to the Secretary of State for Scotland back in 1908 and run by the holder of that post until it was transferred in 1999 from Westminster to Holyrood, where Fergus Ewing, the Secretary for the Rural Economy oversees it today.

Meanwhile the money-spinning bit, the powers over agricultural subsidies and the like – which are always what interest farmers the most – had gone to Brussels when we entered the EU in 1973.

It is this money-spinning bit that will return home with Brexit, and the question is how we should receive it. The Scottish Government asserts the original powers should simply and cleanly be transferred back again from Brussels to Edinburgh. May takes a different view: at least some of the powers, perhaps all – we don’t know – should go to London instead. Why?

The Prime Minister does have a point in one respect. These powers were last in British hands before devolution had been invented, so the UK still had a unitary government at Westminster running a single set of policies regardless of the political complexion of the four nations out in the far blue yonder.

But three of these now have governments of their own, so that the risk opens up of their running sets of policies which vary from or even oppose each others’, let alone whatever might be preferred at the centre. I would not have said that, in the case of agriculture, this prospect was especially daunting.

It is hard to imagine a government in Scotland subsidising, say, sheep so lavishly that it ruins farmers in Wales. In any case, now that we also have the new institution of the Joint Ministerial Committee where delegates from the four nations can meet, there should be no problem in setting one up to deal with details of agriculture.

May’s view is not just different but drastically different from anything so cosy and consensual. In Scotland, we used to say devolution was a process, not an event, and she could now echo this – except that to her way of thinking it is time for devolution to go backwards rather than forwards.

So, much might be gathered from her words to the conference in the context of Brexit: “As we bring powers and control back to the United Kingdom, we must ensure the right powers sit at the right level to ensure our United Kingdom can operate effectively and in the interests of all of its citizens, including people in Scotland.”

But never imagine the Scottish Government as such is entitled to any part in the process because “unlike any of the individual devolved administrations, the United Kingdom Parliament is elected by the whole UK, and the UK Government serves the whole UK … That places on us a unique responsibility to preserve the integrity and future viability of the United Kingdom, which we will not shirk”. In other words, it will be decided in London how, among other things, Scottish agriculture is to be managed in future.

There then need be no recourse to the Sewel Convention, by which Holyrood’s consent was to be sought to any measure passed at Westminster that affected Scotland (though the convention has in any case been dead in the water since the Supreme Court’s judgment on Brexit). Nor can there be any appeal to the historic arrangements that prevailed before EU entry and devolution. Scottish control of Scottish agriculture was so 20th century, don’t you think?

In the 21st century, we will have British control of Scottish agriculture. It would not surprise me to see one agricultural ministry set up for the whole UK. Scots, it will be said, should rest content with the Crofting Commission.

Of course, the ramifications of this go far beyond agriculture – which in today’s Scottish economy is a fairly minor activity, even if practically the sole activity over wide stretches of the country. Also to be returned from Brussels are powers on the environment and on fisheries, and I would have thought the latter case is ripe for a British solution too.

But, over and above the details, the Prime Minister pronounced and reasserted a high guiding principle: the absolute sovereignty of Westminster. In her own code – and she is a politician whose every word has to be watched – she did so in terms as uncompromising as any to be found in the British constitutional history of the last century and more.

Since May took office, we have seen dramatic events in this history which may well have brought to a halt the period of experiment ushered in by the breakdown of Thatcherism and the moral bankruptcy of New Labour. We can no longer delude ourselves with the notion that the UK is somehow painlessly federalising itself.

Today it is the supremacy of the Crown-in-Parliament that has come back with a vengeance. Be very afraid, I would say.

For none of the small nations of the UK can this be a happy prospect. Northern Ireland seems to be getting closer to the point where a democratic majority may decide to take their leave, but for Scotland full national independence looks so much more of a step in the dark. Without it, though, there will be only a deeper gloom in the bleak future of the eternal underling.