I don't know the rules of royal succession, but since the Queen isn't in any hurry to retire, I'm going to guess that Charles might at some point become too old for the job; if that occurs, I think the older son, William, would be king. I wouldn't mind that -- from what I heard Williams is a sensible young man. And he looks a heckuva lot better than the rest of the royals.
There's been some in the past, IIRC - when the heir apparent just didn't want the job or was incompetent. That said, there have been a lot of times in Brit history when they really *should* have skipped a generation and didn't.