Just to clear this up, I realize it's hard to fit a british accent into the Game, but Reeve would be a good choice for a british accent. If you want to voice one of the main characters, you would have to learn more of a north american dialect
Oh, I don't know about that. I don't think having an accent should deter people, let's see what they've got and then we'll decide if it fits or not. They can try both! Besides, the only reason that the characters have North American accents is because the dubbing was done in the United States. If Square had their main English division in Europe, we'd be hearing something much different. Foreign accents make Gaia really seem like a different world. And who says the characters are like us? =) After all, they're originally Japanese. If those voice actors started speaking English, hehehe...
As for Aeris/Aerith, wasn't there a point in the US game when you learned that Aeris' true name was Aerith, similar to learning Red XIII was Nanaki? Their working names stayed the same, as far as I remember. Either way, both Aeris and Aerith are romanized as "e-a-ri-su," pronounced "eh-ah-ree-soo." There's no way to convey a difference between "-s" and "-th" using the Japanese alphabet, so it's mainly a stylization, whatever you choose. Something akin to writing "Katamari Damashii" as "Katamari Damacy." You could even write her name as Earth, as that's what it's supposed to represent.
Personally, I prefer the name Aeris as it rolls off the tongue better and that's how I remember it being in the original game. I realize Square changed it in all media after Final Fantasy VII, but if Square Japan had a problem with how the name was anglicized originally, you'd think they would have said something. I've heard they're quite involved in the translation progress, at least with the later games.