There is no document setting out the detailed principles of a UK constitution. None. There are plenty of books chewing the fat over whether or otherwise there even IS a British constitution, and if so how we might interpret that term considering how nebulous a concept it's become as the 20th century progressed. As to the extensive constitutional reforms brought in by Tony Blair's government - the most radical since WWI, the manifesto on which they were elected - judge for yourself how well those turned out. And I'm still none the wiser as to what those posters continually telling others to 'read the constitution' 😂would like them to take out of this process, or how they anticipate this will change their position and make them ardent supporters of the monarchy. Are there any takers?
As a counter-argument, I could just as easily urge you to look up the term 'elective dictatorship' or to read the Irish Constitution (unlike the British one you can read it in full, including all articles and amendments), especially those relating to the sovereignty of the republic, separation of powers, and rights of the Irish people. As a system of liberal democracy it's an effective model. Curious as to how these points might compare to the issues with British one you are claiming to know all about.
It's not only the Windsors, who are a small aspect if an influential figurehead of a dying system, around which questions are being asked about a UK constitution pretty much in its decline. To the extent that we do have a set of rules around which government is based, we have constitution (for what it's worth). But even academics are divided over its meaning. The traditional principles of state and national identity - that 1000 years of 'history' monarchists bang on about - is pretty much redundant given every government seems to lack faith in its own principles and seek to 'modernize' it at every given opportunity. And now we've even seen fit to remove the checks and balances afforded us by the EU a radical reform of the whole system is even more urgent.
As well as '1000 years of history have worked perfectly well, it provides stability' [it doesn't, for above reasons; that is a comforting illusion], monarchists like the argument that it would be a terrible idea to overthrow the system without complete clarity as to what its benefits would be, and the exact structure of what replaces it. I agree completely. It needs a gradual transition and careful consideration, not an idealist revolution. That's a discussion which was always going to begin with the death of Elizabeth II. And those questions are starting as of now.