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

If so, then it is up to the person claiming that to provide a cite. Not demand that others provide a cite.

Most of the governments of most of those “other democracies” could be better described as military juntas, no?

Perhaps at some point in their histories, which is where the cliché of Latin America = military junta comes from, but I’d say those days are over. There’s a tendency for politics to get very divisive and occasionally violent in Latin America, but most of the countries have decent democratic structures by now, and the cliché military junta commonly associated with Latin America is a thing of the past. Not even Maduro fits this picture.

I believe it’s just a matter of custom, not established law. There’ve only been three times that the monarch’s regnal name hasn’t been their first name, and on every occasion the new monarch used one of their other baptismal names - Alexandrina Victoria became Queen Victoria, Albert Edward became Edward VII, and Albert Frederick Arthur George became George VI.

(Also, Robert III of Scotland was born John Stewart, but I’m not sure that counts.)

ISTR it being speculated at one time that Charles would choose to become George VII when he became king, but I guess his grandson will get that honor instead.

When I asked for a cite, I was asking for a cite of a British or English monarch not reigning under their baptismal name. With the monarchy, much is constitutional custom, based on a millenium of reigns.

To the best of my knowledge, the only monarch who did not reign under his baptismal name was Robert III of Scotland (r. 1390-1460). HIs was an odd reign, where he changed his name to avoid links to an English puppet whom the English tried to install as King of Scots; Robert was also effectively pushed out of the monarchy for several years.

As with many things in British constitutionalism, there is the law, and then there is constitutional convention. Could the Supreme Governor of the Church of England abandon his baptismal name, given to them in the C of E sacrament of baptism? The fact that no English or British monarch has done so in 1,000 years is a strong indication that the answer is “no”, just as the fact that the monarch can refuse assent to any bill, but hasn’t done so for 300 years, is a strong indication that the convention is “no”.

Without a clear precedent of an English or British monarch abandoning their baptismal name, I don’t think it’s correct to say dogmatically that they can reign under whatever name they choose. British monarchs are not popes.

The monarch is not a manager of a McDonalds; different rules apply.

For instance, a manager of a McDonalds can quit their job on a moment’s notice and walk out.

The King is assigned his job by statute and needs an act of Parliament to quit.

No, that’s not correct.

The House of Lords and the House of Commons were convened by William of Orange, who was not king, but had invaded England at the request of Protestant opponents of James II. It therefore was not a Parliament, since it had not been convened by the monarch, and the monarch (James II) was not present).

The two Houses issued a declaration that James II had abdicated, set out in the preamble to what became the Bill of Rights:

And whereas the said late King James the Second haveing Abdicated the Government and the Throne being thereby Vacant ,

The two Houses then offered the throne to William and his wife, Princess Mary, to rule jointly, provided they accepted the restrictions on monarchical power set out in the Bill.

William and Mary accepted the throne on those conditions, and then gave royal assent, giving the Bill of Rights legal force.

This was a very legalistic revolution, but a revolution nonetheless, since the constitutional principles governing the monarch and future parliaments were changed by a mechanism inconsistent with the constitutional principles of the day. Hence the name, “Glorious Revolution”.

I think this is probably a sticking point for people from the US to understand. It’s not that we don’t have customs - we do. It’s just that a custom would never lead us to answer a question about could something happen or is it legal for something to happen with “It hasn’t happened in 1000 years so the answer is no”. Especially not now. We might say it’s never happened or that it’s customary to do whatever the custom is - but the direct answer to a question about breaking a convention or norms is “There isn’t any law/rule against it”

OTOH, the Catholic Church, whose theory of sacramants is AIUI pretty similar to that of the C of E (I guess the C of E did not ditch this part of Catholic teaching during the Reformation), has no problem with a newly elected pope substituting a new regnal name for the baptismal ones. This would support the view that there are no major theological concerns against a name change.

Also, it seems that the C of E is pretty relaxed about the possibility of a change to a baptismal name; this requires confirmation by the bishop:

If it is desired for sufficient reason that a Christian name be changed, the bishop may, under the laws of this realm, confirm a person by a new Christian name, which shall be thereafter deemed the lawful Christian name of such person.

Source

I don’t know if this means confirmation in the narrow sense, i.e. the sacrament of confirmation, and can be done only on that occasion, or whether the provision allows for name changes throughout the individual’s life.

A bit of context: since the Conquest in 1066, there have only been nine names used by the Kings of England and then Britain.

Of those, two were only used once: Stephen and John.

The last time a king came to the throne with a never-before-used name was in 1715, when George of Hanover became king.

For queens, there have only been four names used: Mary (twice), Elizabeth (twice), Anne and Victoria.

So, a total of 13 names used by the Kings and Queens of England and then Britain, for close to a millenium.

And all of those were their baptismal names.

Good point, but several had many choices in their baptismal names- Charles III (Charles Philip Arthur George) for example.

What about George VI? Wasn’t he Albert before he assumed the throne (and then decided King Albert was right out)?

He was baptized Albert Frederick Arthur George.

Yes, as discussed above at posts 24 to 26.

Wasn’t George I technically baptized “Georg”?

He probably was, but at that time (and long thereafter) it was common practice to translate names between languages. “Georg” and “George” would not have been considered different names but different language versions of the same name.

What if Prince William’s first son dies (sorry, never can keep their names straight), his daughter eventually marries and takes her husband’s surname, and in the fullness of time inherits the throne? Then you might have a Queen with the surname of Gonzales or Cho or anything at all.

Or make a completely new name under the Royal Proclamation just like George V did in creating the House of Windsor.

That situation would be analogous to Elizabeth II, who decided to keep Windsor as the name
of the royal house.

EII did that knowing she was the heir. If Charlotte gets married and takes a new name and then after that George dies before having kids, it’s a different situation.