My MC has a distinct British accent in my head. He's from England, but it's not exactly the most important thing in the story. Everyone else is American, however. Should I mention the accent or just leave it up to the readers to assume that since he's from England, he's going to have an accent?
I think either could be made to work. Readers will probably assume he has it if it's known he's from England. However, I think a mention could be made if it's worked in properly.
If the region he comes from is somehow of relevance in the story/ for the character, then yes. What kind of accent were you thinking?
there are many 'english' accents... some are regional and some are social class-based... you will have to specify which one your character uses... and it's more than just the pronunciation of words... you'll need to do your homework and research the idioms and other regional/class differences to make his dialog sound authentic... an easy way to do this is to think of a movie character you want him to sound like, then download the screenplay to see how the dialog is written... and do the same with novel characters [in books by good writers]...
Yeah, I was thinking of a Birmingham accent. I don't know how relevant the accent would be to mention, though. I don't want it to come out as "he said in an accent that brought memories of her summer trip to England to mind". I don't want to draw that much attention to it, but I want people to realize the accent is there.
It can be a bit of drag (for reader and writer) going down the 'must try and faithfully recreate this or that accent at all times' avenue. Personally, I would make little of it. Do you know the accent much? Cat Deeley has retained some of her Birmingham accent and I think she presents shows over with you.
You could make your character use a few typical British words or expressions (eg, "pissed" in the sense of being drunk, "mad" in the sense of being angry, "being sick on" in the sense of throwing up, "chatting" in the sense of having a light conversation), and the reader will hopefully be reminded that he speaks British English and imagine his voice that way. Just don't overdo it.