What would have to happen for an upcoming British monarch to not be of House (Mountbatten-) Windsor?

Cite, please? See discussion starting at post 23 of this thread.

Just on principle or would the monarch have to refuse royal assent on a major bill? Like if Charles III refused royal assent on giving £1000 to Miss Polly’s Home for Precocious Pussies, do you really think that they would end the monarchy?

It is one of the royal prerogatives. MacCormick v Lord Advocate does not exactly cover the name (just the number) but it is certainly close.
If you don’t believe it, how about you cite the law that says the British monarch must choose a regnal name from their given names?

If the bill to give £1000 to Miss Polly was duly passed by the Commons and Lords (or the Commons alone, under the Parliament Acts 1911 and 1949 which state that the Commons alone can submit a bill for royal assent), and the King refused, then yes, that would absolutely be a major constitutional crisis that could bring down the monarchy. It’s not the substance of the bill that matters; what matters is that the King acted against the principle that royal assent is never refused if the bill has duly been passed. This principle is so fundamental to British democracy, and any breach of it is so unthinkable, that in case it really happens, all bets are off.

IIRC, there was a bit of an end-around where Parliament declared that James had not abdicated but by voluntarily exiling himself it was OK to go on without his assent. There’s a little more technical legalese but that’s the gist of it.

This is what happened the last time RA was refused. A bill passed under one PM was refused RA on the advice of a new PM.

I’m not sure I follow. The last time royal assent was refused was in 1708 by Queen Anne. This was before the office of prime minister in the modern sense arose.

Looked it up and I was almost correct. It was on the advice of her “ministers”

I think it’s because we don’t understand the purpose of having what is essentially a rubber stamp. If the monarch always assents if the bill has been passed , then I really don’t understand how that’s different from a head of state who has only a symbolic/ceremonial role whose assent isn’t required.

In practical terms it comes down to the same thing, of course. But the monarchy wasn’t designed from scratch; it evolved over centuries from a time when it held actual power to its present role. So the power of the monarch to grant assent has been preserved, but only as a constitutional fiction that maintains the resemblance of actual decision-making functions of the monarch.

Well, why would the King not have the same rights over his name as everybody else? The new manager of a McDonald’s could unilaterally adopt a new name to commemorate that promotion and it would be totally legal AFAIK.

I see that no one has used a different name than one of their birth names (at least since Anne which is as far as I’ve looked). That doesn’t mean that they have to do so.

This is an example of how unwritten constitutional conventions develop. So far no monarch has attempted to rule under a name that was one of the birthnames, so no rule in this regard exists. As soon as someone tries to do that, we’ll see if it works or not, and that will then set the precedent, and hence the rule, for the future.

So is this the royal brother Rupprecht?

Any person in Britain can change their name at will by just using it. The only caveat is that for to change some documents like passport or driver’s license you may need a deed poll. And here is my cite. So why would a monarch be treated any differently?

It’s not really the same thing. They aren’t doing an official name change. They are going by a different name “professionally”.

It’s not just constitutional monarchies like the UK, Spain, Belgium, Netherlands, Norway, Denmark, Sweden and Japan that have a system like this, it’s almost every western style democracy in Europe and parts of the Commonwealth that has a Prime Minister.

So that includes Ireland, Germany, Israel, India, Italy, Pakistan, Hungary, Austria, Poland, Portugal etc., etc., etc., All of these countries have a president, who you probably haven’t heard of and very rarely makes the news outside their own country, who is head of state. The head of government is the Prime Minister (or Chancellor) who has a relationship with the president that is fairly similar to that between a PM and their monarch.

In fact in western style democracies, it is the USA which is unusual in having an executive, political head of state/president. France is the another outlier, where they have a prime minister, but the executive president is more powerful.

Any person in the UK can change their religion at any time. But the monarch can’t, at least not to Catholicism. So perhaps there are special rules applying to the monarch also for name changes?

This statement is a bit too strong. The American presidential model has been copied by lots of other democracies. Almost all of Latin America follows it, for instance.

Yes, I realise Latin America follows the American model in general - that’s why I qualified it with “in Europe and large parts of the Commonwealth” in a previous paragraph. I failed to do that in the paragraph you quoted.