>In the UK, a central principle is that parliament is entirely sovereign. //
Not entirely sovereign, legally the Sovereign is advised by parliament, the government is hers, and she signs acts in to law according to her whim ... of course the power resides in a delicate balance and the legal situation belies the complexity of the true power structure.
This is sort of true, but only in extremely limited cases.
It is a constitutional convention that the monarch will follow the advice of ministers other than when exercising reserve powers, and ministers have extremely limited constitutional flexibility in terms of advicing the monarch to give assent. The monarchs power to refuse royal assent for bills is not part of the reserve powers other than in very limited circumstances (she can delay assent in the case of "near revolutionary" conditions; she can in theory refuse assent on advice of ministers, last exercised by Queen Anne in 1708; she can refuse assent on bills that directly relate to her reserve powers themselves, or her property).
So while on paper she "signs acts into law according to her whim", if she fails to sign laws outside that very narrow scope, she'll have violated a constitutional convention that has been a settled part of UK constitutional law for something like two centuries.
This would not just be outrageous, but would be subject to legal challenge, as notably, it has been established since 1611, and not challenged by a monarch since 1688, that the judiciary has the power to restrict the royal prerogative, and the judiciary in the UK has centuries of history of taking a very dim view on monarchs trying to reclaim powers that have been de facto transferred to parliament. Odds are she'd find herself having the power legally stripped from her at best, at worst it'd be the end of the monarchy.
In other words, in practice it is legal fiction that this power rests with the monarch - outside of the reserve powers it rests with the monarch only as long as she only exercises them in a ceremonial capacity in line with the wishes of parliament.
Even the reserve powers are retained mostly because they are exercised so incredibly rarely (e.g. some of them have not been attempted used for centuries) and/or have very limited effect (e.g. on the rare circumstances where the monarch has exercised her powers to choose whom to appoint as PM in modern history it was accepted because the decisions were in line with parliamentary reality, and because parliament could instantly overthrow said government if it did not agree with the choice), but the reserve powers are at least nominally actual powers that the monarch can with some caution actually exercise and get away with it. The rest of the royal prerogatives are not.
Not entirely sovereign, legally the Sovereign is advised by parliament, the government is hers, and she signs acts in to law according to her whim ... of course the power resides in a delicate balance and the legal situation belies the complexity of the true power structure.