So the BBC is running a story right now asking just why it seems so many Fantasy characters speak with distinctly British accents. Speaking strictly as a fan of the Fantasy Genre, It is a rather preposterous question to be honest. They act as if they need it spelled out to them that the reason the bulk of the characters in Middle Earth and Westeros speak with a variety of British accents is because, well, they are set in Pseudo Britains. They argue, rather unconvincely, that due to Game of Thrones being based on an American series, the characters should probably have American accents. I could buy this if the series being adapted was Terry Brook's Shannara, set unquestionably in a future north America. In fact, I would be offended if Shannara was adapted and wasn't populated by North American accents. If they ever get around to adapting Robert Jordan's The Wheel of Time, the author himself was explicit on what sort of accents his characters should have, and I hope, for sake of the strangeness of seeing a bunch of African Samurai's speaking with Texas accents, they honor the author's wishes.
The Article goes on to question whether or not the surge in British Accents in Fantasy is a result of Kevin Costner's abysmal accent in Robin Hood: Prince of thieves, It's an alright question but rather dubious. Fantasy Films have been sporting accents and casting "Foreign" (Non American) Actors for a very long time. Or that perhaps it is in reaction to years and years of British accents being synonymous with villainy in Hollywood. Star Wars being a great late example of this. But Practically every World War II film that didn't force its actors to use bad fake German accents, simply substituted for British.
However, that being said, if you look at the 1977 Rankin-Bass adaption of The Hobbit, the entire cast, more or less, is comprised of Americans, and it worked. At least to me, I felt the voices in The Hobbit were all quite good for the characters. They were believable because it was consistent. Obviously you run into problems when you have a hodgepodge of accents with no discernible reason for them to be different. This was a sevre problem with the recent Tom Cruise film Valkyrie. Every other actor in the film, except Cruise, had a British Accent. It made the Germans convincing, except for Tom Cruise. Why so many Americans, myself included, have so much difficulty with accents, baring caricatured ones, is a mystery.