Kinda straightforward… but what happens if Prince Charles suddenly, and unexpectedly passes away (heart attack, double-decker bus attack, abduction by aliens attack, etc…) leaving her on the throne? On her passing, does the Crown pass to William without any problems?
What if Charles had a surviving brother? Would the line of succession then jump from William to his first cousin due to the surviving sibling?
I do not need an answer fast. This is merely out of late-night, “insomnia-related reading.”
Seriously, I’ve never even been to the UK.
No: the line jumps from the end of William’s line, to Henry’s line all the way to its end, to Andrew’s line all the way to its end, to Edward’s line. To jump from the end of Henry’s line to Andrew’s descendants, Andrew would have to be deceased. To jump from William to a cousin would require quite a pileup of corpses: William’s kids, Harry and kid(s), Andrew (and kids, and Edward, in order to pass to one of Edward’s children).
To the OP’s original underlying question - it isn’t a situation similar to, say, Saudi Arabia, where the successor is open to debate and perhaps only one person identified - the course of the succession is set. If a terrorist wiped out 50 royals at a wedding, there’ll already be someone in 51st line to the throne who would step right in.
ISTR that particular situation hasn’t seen an actual ascent to the throne, though. Pretenders, yes; an actual new Tsar, no. And it still was only the Tsar’s nuclear family that got executed. There are a lot of actual changes of dynasty throughout European history (and, I’m sure, those of other places) which would apply more closely, but they often involve children dead of natural causes rather than massacres.
Historical note: Victoria’s uncle, King William IV, was King of both the U.K. and Hanover, as were his brother and father. But when William died in 1837 the thrones of Britain and Hanover were split apart: Victoria got the U.K. but her uncle Ernst August took the throne of Hanover. This was due to a difference in inheritance rules: Britain used agnatic-cognatic primogeniture, while German states used Salic law with females almost ignored.
There’s an earlier example. Edward III’s eldest son was Edward the Black Prince. He died before his father, and in 1377 Edward III was succeeded by his grandson, the Black Prince’s son, Richard, who was 10 years old.
So Richard II took precedence over his uncles - John of Gaunt, Duke of Lancaster, Edmund, Duke of York, and Thomas, Duke of Gloucester.
Richard II’s reign went badly, but John of Gaunt remained steadfastly loyal to his nephew. However, after the death of John of Gaunt, his eldest son deposed his cousin and became Henry IV.
The whole story of the disputed succession of the houses of Lancaster and York after Richard II is long and complicated, and it would be possible to spend all day arguing about it.
But the part that’s relevant to this thread is that technically the next in line to the throne, after Richard II was forced to abdicate, was a descendant of John of Gaunt’s older brother Lionel, Duke of Clarence - Edmund Mortimer, Earl of March, another young boy.
Henry, the son of John of Gaunt, was only second in line to the throne. Whether Parliament had the right to ignore the line of succession is another question, but that’s what they did. They were fed up with Richard II, and wanted Henry on the throne, not another boy king.
Later that was the basis of the House of York’s claim to throne.
Which is too bad, because they could use a bit more variety in the names of British kings. Something other than Charles, Edward, George, Henry, James, Richard, or William (which all but two kings have been named since 1066).
Part of that is this tendency during the current dynasty not to use the first names they were given. Both Edward VII and George VI were named Albert, and known as “Bertie” among their friends and family, but chose another name at their coronations out of respect for the late Prince Consort. It looks like Charles intends to do the same and become another George (which makes me wonder why the Queen didn’t just name him after her father if that was always the intention).