The 1960s Tory prime minister Sir Alec Douglas-Home - a Scotsman to his bones in spite of his plummy Old Etonian accent - was one of the most maddening figures in Scotland's modern story. A sincere devolutionist, he nevertheless played a crucial role in thwarting an elected Scottish Assembly in the 1970s by saying something that he must have known or strongly suspected to be untrue - that if Scots rejected Labour's blueprint for devolution, a better version would swiftly be implemented by the incoming Thatcher government. Partly as a result of those words, Douglas-Home didn't even live to see devolution finally become a reality - 20 years later, under a Labour government.

Not long before he died, he gave a hint as to the thinking that guided his seemingly irrational actions. He implored his fellow Scots to accept the overwhelming importance of the United Kingdom - and stressed that only after everyone had committed themselves to the UK could there be a civilised internal discussion about how power should be dispersed within these islands. What that doctrine seemed to amount to in concrete terms was that Scotland had the right to ask England's political representatives (usually Tories) for the opportunity to make some of its own decisions, but only if it accepted that London Tories had the absolute right to say "no". And, as the Thatcher example demonstrated, the answer usually would be "no".

Theresa May's haughty, dismissive, sneering posture towards Scotland since the Brexit referendum has been the Douglas-Home doctrine without the honey, and on steroids. Because Scotland narrowly voted No to independence in 2014, it has apparently forfeited all say on its own future. It has accepted the overwhelming importance of the United Kingdom (that might be news to a good few reluctant No voters), and must forever more go wherever the UK goes, and accept London's final say on all matters.

It doesn't matter that Scotland voted by an almost 2-1 margin to remain within the European Union. The UK has decided to leave, and that's that. There's no room for a compromise that would at least allow Scotland to remain within the single market, in spite of the fact that not even England voted to leave the single market - no, the London government has unilaterally decided that it's to be a Hard Brexit (sorry, a "red, white and blue bespoke Brexit"), and Scotland is to be taken along for the ride.

Some may have naively wondered whether the Douglas-Home doctrine was being very slightly undermined last year when Theresa May declared that there would have to be an agreed UK negotiating position on Brexit that took Scottish concerns fully into account. A few commentators noted that this appeared to amount to a Scottish veto. But they reckoned without May's boldness in taking the Douglas-Home doctrine to a whole new dimension.

As her statements in recent days make clear, she always reserved the right to decide for herself what Scotland thinks and what is best for Scotland. So the negotiations to decide upon an agreed UK position were actually negotiations with herself. No wonder they went so smoothly, and no wonder "Scotland" is so delighted with the outcome. The real Scotland and its elected government were never more than bemused bystanders during the process.

The final insult has now been delivered by the Scottish Tory leader Ruth Davidson, presumably acting on the instructions of her London colleagues. She announced during a radio interview that powers over Scottish agriculture and fisheries will be 'repatriated' from Brussels to London after Brexit, and only after that can there be a discussion about whether some of them should be passed on to the Scottish Parliament.

Douglas-Home would doubtless be purring with approval, but there's a snag - agriculture and fisheries are already devolved as a result of a 'discussion' that took place many years ago. Under the terms of the existing devolution legislation, all of those powers will automatically transfer to Edinburgh upon departure from the EU. If that is to somehow change, the current devolution settlement will have to be ripped up, and the Sewel Convention that is its cornerstone and safeguard will have to be utterly disregarded. And yet there's every indication that the Tories are proposing to do precisely that.

It seems that under the latest adaptation of the Douglas-Home doctrine, past agreements on the dispersal of powers within the UK aren't worth the paper they were written on. London can call back powers at any time, and insist upon starting the whole 'discussion' process from scratch - with "no", as ever, being the most likely final verdict.

Little wonder that Nicola Sturgeon has finally decided that enough is enough, and started pointing towards the elephant in the room - namely that the Tories simply have no mandate to govern Scotland, to speak for Scotland, to decide what type of Brexit is best for Scotland, or to decide that Brexit itself is in Scotland's interests. Scotland has its own voice, and it is pro-Europe, pro-single market, pro-self-government, and anti-Tory. As a recent Panelbase poll demonstrated, it is also narrowly in favour of a second independence referendum in the relatively near future.

Scotland has its own voice, and it is pro-Europe, pro-single market, pro-self-government, and anti-Tory

Sturgeon is now advancing the Douglas-Home doctrine in reverse. The existence of the United Kingdom is not the principle that everyone must agree to before discussions can even begin. Instead, the point of fundamental importance is that the Scottish people are sovereign and have the right to determine their own future. Once that is accepted by everyone, including Theresa May, then we can have a civilised discussion about whether a constitutional relationship with the rest of the UK should play a part in Scotland's future.

Our view on that point might even evolve as circumstances change. Why shouldn't a sovereign people that narrowly chose to retain ties with the UK in 2014 rethink that decision when it emerges that being part of the UK is incompatible with membership of the EU? And why shouldn't a sovereign people decide for itself the date on which that decision should be revisited, rather than having to go cap in hand to a politician in another country who has no relevant mandate? Nobody in England would have accepted that Brussels should be able to decide the date of the UK's referendum on the EU - its second, not its first, remember - or that 'Eurocrats' should have a veto on whether it even took place at all. Why should a basic democratic principle work any differently in Scotland?

The Tories have over recent months indulged in ever more elaborate logical gymnastics to cast doubt over Sturgeon's cast-iron mandate to call a referendum at a time of her own choosing. But a UK government that has just one MP in Scotland and is dragging Scotland out of Europe against its will is always going to be on the losing side in any contest between mandates. Sturgeon has just flipped the kaleidoscope - and the prime minister is unlikely to appreciate the view from her new end.

James Kelly is author of the Scottish pro-independence blog, SCOT goes POP! Voted one of the UK's top political bloggers, you can hear more from James on Twitter: @JamesKelly