That's true, but PMs are motivated not to "embarrass" the monarch if it can be at all avoided. It would be a very unpopular move, as it was when Boris Johnson "

That's true, but PMs are motivated not to "embarrass" the monarch if it can be at all avoided. It would be a very unpopular move, as it was when Boris Johnson "advised" the late Queen to prorogue Parliament during the Brexit period. The PM would almost certainly come out worse unless the monarch really was up to something quite bad, and building on Crown land mightn't be bad enough.