The reality is somewhat more complex once you consider the UK's treaty obligations, many of which are hard to reconcile with "pure" Dicean sovereignty, but it's a good starting point. As a counter example, the UK can't practically legislate in a manner that breaches the ECHR (European Convention on Human Rights - completely unrelated to the EU). Similarly, while still a member of the EU the UK, like all EU members, has to accept the primacy of EU legislation over domestic statutes in the areas in which the EU has the competence to make laws.
Studying the uneasy relationship between dicey style sovereignty and EU legislation (hello "Factortame" cases...) was a core component of a law degree in the UK for the past few decades, I assume this will change shortly given recent history!
So, yes, it's more complex. I don't think anyone would suggest parliament would just rewrite arbitrary laws on a whim, because there are complex dependencies (hello Brexit). The point is there is no legal hindrance to it, and that is almost unique (I write "almost", but I don't know of a single other country where parliament has so unfettered power - I'd love examples).
> If the time should come when our Parliament deliberately passes an Act with the intention of repudiating the [EU] Treaty or any provision in it or intentionally of acting inconsistently with it and says so in express terms then I should have thought that it would be the duty of our courts to follow the statute of our Parliament.
or see Thoburn v SCC which established that constitutional statutes (such as those establishing the primacy of EU law) are only protected from _implied_ repeal.
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.
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.
Most places there are constitutional limits on what parliament can decide and/or qualified majorities and other constraints needed to change constitutional law. In the UK, a central principle is that parliament is entirely sovereign. That is, parliament is not bound by any law decided by past parliaments, but can revoke or alter any of them (they are bound as long as they let a law stand, though) with a simple majority.
While your comment elsewhere that they need things to pass through House of Lords is sort of right, consider that the House of Commons powers are bootstrapped - e.g. the UK Parliament Acts 1911 and 1949 severely curtailed the Lords powers, and both were passed as a direct response to the House of Lords being seen as obstructing the House of Commons. The Lords failed to stop both, because they knew in advance of the 1911 act that the government had threatened to simply stuff the Lords full of sympathetic new peers (lords).
As such, while the UK has a bicameral system, the Lords exceedingly rarely stop laws, it usually only delay them or cause negotiations over revisions, as if the Commons decide to push something through over the objections of the Lords, they generally get their will, and coupled with the lack of constitutional hindrances, they're more powerful than even most unicameral legislatives.
> I interpret this as trying to understand the problem better, not prosecute someone, in which case it's not who's-legally-responsible, but who-gets-Facebooks-DNA-and-vision-for-the-future.
This is true. It's not in any way a court proceeding. In this respect it's no different from being summoned to attend an inquiry in any other parliament.