There are several English actors in the new Star Wars trilogy: Daisy Ridley, Gwendoline Christie, and John Boyega, just to name a few. Some of them use their normal accent, or something close to it. However, for some reason Finn has an (American?) accent, rather than John's natural accent. Why?
He tried with a British accent first, and JJ Abrams said it didn't work.
Straight from the mouth of John Boyega himself in this interview on the Graham Norton show:
This pretty much settles the matter for John Boyega / Finn specifically. But for more about the in-universe significance of British and American accents in Star Wars generally, see the following Q&A from another SE site devoted to sci-fi and fantasy: