In the MTV video, Lily said all the Shadowhunters were British. I get they're all from Idris technically, but I never thought they had accents of any sort, wouldn't Clary have mentioned that? Or did she mean the actors... I'm so confused!
I think I’ve always left it pretty open in the books as to how the characters sound. Clary never comments on it; I’ve always thought there was likely an Idrisian accent but never wanted to heavily describe what that might sound like — I think the best way to treat dialect is really really lightly and the best way to deal with fantasy accents is to let readers imagine what they might sound like.
Books are a very different art form than movies. What would take pages of description in books can be served with visual or audible shorthand in a film.
The decision to have the Shadowhunters speak with British accents (their native accents in the case of all the actors pretty much except Kevin) has to do with Harald wanting to show that they are different from those that surround them, that they are a people who are separate, different, and with an older heritage. Maybe a good example of something like this is that in Lord of the Rings, Sam speaks with a working-class Gloucestershire accent and Frodo speaks with a posher British accent because it’s fast audible shorthand for the class difference between them. The Gondorians speak with a more Northern accent to underlie that they are from an older, remote society. None of this is in the books but it tells us a huge amount about the characters that LOTR takes many pages to establish.
Anyway, this is just to say that: movies take visual and audio liberties with text because they have a different way of communicating the same things books communicate, because they’re a different medium. I actually do like the Shadowhunters’ British accents (they all did their auditions with both British and American accents and the British accents for whatever reason worked better) – and I don’t think it’s contrary to the books’ spirit, but YMMV!