Fun fact: If Prince William becomes king, his regnal name will be William V of the United Kingdom of Great Britain and Northern Ireland

Provided that either of those states still exist by that point. If Northern Ireland goes back to Ireland and Scottish independence ends the United Kingdom, then he would just be William V of England.

Or maybe William the Bald
would be more appropriate?
(via Wikipedia)

If Prince Harry somehow, someway were to become king, his name would be Henry IX and would be the first King Henry since 1547!

Hopefully he'll avoid the
relationship issues of his predecessor.
(via Wikipedia)

William's son, George, would be known as George VII.

Prince Charles, meanwhile, would be Charles III. Hopefully, he won't be like Charles I, who ended up losing his head - literally - over a change in management, nor Charles II, who was partly responsible for his successor, James VI being overthrown in the Glorious Revolution by...William III.

Let's be real though,
this guy is going to be senile or dead by then.
(via Wikipedia)

And what of the Duchess of Cambridge? If her husband were to ascend the throne, she would be known as Katherine, Queen Consort.