And if both Charles and William predeceased the Queen, George would be king. And if that happened while he is a minor, he would need a regent, which under present rules, is the person over the age of 18 who is closest in line to the throne. That person would be Harry.
As for accents, it's just as valid, especially for someone from another country, to talk about a "British accent" or an "American accent" as an umbrella term. I think most Americans recognize that there are different accents in the UK, but most of us can't differentiate among them all that well. So we need an umbrella term. I think the same is true for most British people and American accents. I, as an American, can differentiate many "American accents" that people from other English speaking countries probably can't. So they need an umbrella term.
Plus it's likely that when an American uses "British" it signals that they probably do understand the basic make-up of the UK. A couple of years ago, I overheard a woman speaking in a "British accent" that some people would say was an "English accent" and when I struck up a conversation, I discovered she was Scottish. Very posh Scottish as it happened because she was speaking in an RP accent.
"British English" and "American English" are linguistic terms of art. Using similar descriptors for accents is closely related to that.