Well, a lengthy (possibly even multi-generational) regency becomes a possibility if, for good reasons, William’s heir is not declared dead. For example, his kidnappers might continue for decades to occasionally communicate, and even furnish reasonable proof, that he is alive. Or they might make credible claims that he’s alive, that they have no intention of killing him, and that they will keep his identity as heir a secret even to him, and then cease communicating altogether. In such scenarios, who’s to say the regency wouldn’t continue indefinitely? Of course, the royal family would probably be plagued with phony claimants, no matter how trivially disprovable, forever after…
Under UK law, it’s not possible to have the monarch declared dead. (Presumption of Death Act 2013 s. 31(2)). Anyone else, yes, but not the monarch. Presumably the thinking is that if the monarch has disappeared it’s for Parliament to decide when the time has come to presume him dead, not the courts.
Under the scenario outlined in the OP and given current UK law, the situation before the kidnapping would be: (1) Charles would be king. (2) Assuming Charles had been declared medically incapacitated, William would be regent, advised by councillors of state who would include his wife, his children (if over 21) and his brother.
Upon William’s kidnapping, he could no longer serve as regent. Although the monarch can’t be declared dead, the regent can. But it takes considerable time for a declaration of death to be made, and I doubt they could wait that long. There is provision in the Regency Act to “depose” a regent if he or she is “unavailable for a definite cause”. I expect that this would be done, and the regency would pass to the first person in the line of succession who was available to serve - in the scenario outlined, Harry.
So Harry would serve as regent for Charles, who would still be king. On Charles’s death, he would continue serving as regent, but for William. At some point Parliament might intervene to formally confer the crown on Harry. But it would take a parliamentary intervention.
Thank goodness. The Royal family is enough of a circus without adding in the Osbourne factor.
What relevance does the plot of that film have to the question? Nobody got kidnapped or incapacitated in that film, and Goodman’s character didn’t have any illegitimate children who may have wanted to move into Buckingham Palace. As I recall, the premise was quite simply that all the heirs in the main bloodline died, and so the throne immediately passed, without any legal complication, to someone much lower down in the line of succession.
But it says HER MAJESTY, which seems to me to be merely referring to QE II, as the person giving royal ascent.
It seems that she wouldn’t give ascent if it said that SHE HERSELF would be declared dead by the courts automatically, but she is happy for her ancestors to be summarily declared deceased. This has happenned in numerous other laws, I recall. she considered herself impeccable and god like, but her ancestors will have to smote the law with crushing blows to get the same for themselves.
I’m pretty sure there’s a law somewhere saying that a law referring to “Her Majesty” or “The Queen” (or the male forms) also applies to her successors. It would call her something like “Her present Majesty” if it only applied to her.
OK, but let’s complicate things.
William et al are kidnapped. Rogue Sealand Nationalists or something.
Harry becomes regent. Kicks some ass at it. Scores the winning goal to bring the World Cup to England. Spends a series playing the Doctor. Best. Regent. Ever.
Eventually parliament acts, William et al are declared dead and Harry gets his big day in Westminster Abbey. Good on him. He and his heirs fill the role through peace and war until, say, 2112.
Suddenly an heir to William pops up on the scene. He’s escaped from Sealand and identified himself to the Virtual Reality Daily Mirror or something. It’s incontrovertible. DNA confirms it. He looks like William and so forth. The current monarch, Harry’s heir, acknowledges him as cousin and so forth.
What the heck happens then? Is the new guy suddenly in line to become King or what?
It depends entirely on what Parliament put in the act declaring William et al to be dead. A well-written law would deal with that, either by excluding William’s line forever or explicitly stating what should happen if he or his progeny ever make it back.
I am hardly any kind of expert here, but … unless the law declaring William dead were to specifically leave the door open for surviving descendants discovered in the future, then any descendants of William would be permanently not in line for the throne on the grounds that, at a time when the crown changed hands they were “dead”.
I put dead in quotes there not because they were just legally dead and not actually dead, but because succession considers a lot of people to be “dead” who aren’t really dead. For instance, anyone who hs ever been a Catholic is to be considered “dead”. (Their children can inherit but they cannot.)
I have re-thought my position: I think William’s descendants would still be in line, but only if Harry’s line died out. With no heir, the search would go back to “other children of Charles”, which would bring William back into the picture, and down the line to his descendants.
Re-thought it again, I’m back to my first position, which was that I don’t have enough expertise to have an informed opinion. 
Brilliantly written.
Depends, as Lord Feldon says, on what is said about this in the Act that declares William and his issue dead, and makes Harry king.
My will guess is that the Act would absolutely and permanently bar William and his issue from the throne, on the basis that (a) above all, we want certainly and clarity about this, and (b) if they ever do turn up, then and only then is the time for Parliament to consider whether they should be immediately put back on the throne or reinserted into the line of succession, and enact legislation to do so. If you preserve any rights for them at all when they are presumed dead, you are just creating incentives for imposters to appear. Why would you want to do that?
Or to avoid the political fallout from his Queen Consort being a divorcee, as was the case with his grandpa’s brother. The founding principle of the CoE – so the king can get a divorce – apparently only applies if you’re already king.
The biggest objection to Wallis Simpson wasn’t that she was a divorcee; it was that she was an American divorcee, still married to her second husband, carrying on with Edward while simultaneously involved with other men (including possibly a high-ranking Nazi), with a scandalous reputation and uncertain politics. By the time the scandal broke in Britain, Edward was already king, and the British establishment didn’t think much of his work ethic, politics, or understanding of the role of a constitutional monarch anyway, and such a dubious woman as Wallis was the breaking point.
I’m curious about the religious aspect – the king/queen is required to be a protestant?
What happens if QEII sees the light and suddenly declares she’s a Buddhist? Or an atheist, for that matter? Does someone rush in and grab the crown off her head?
The only thing forbidden in law is for the monarch to be a Roman Catholic. There’s no prohibition on any other faith.
Is it Roman Catholic that’s specifically excluded? Could someone from one of the Orthodox Catholic churches be monarch?
This is the 1701 Act of Settlement’s line:
So adherents of the Romish Pope specifically.
Yes. The CofE self-describes itself at “catholic and reformed” mind you, just not in communion with Rome.