Why do people in fantasy worlds speak with a British accent?

This is something I’ve often wondered, especially lately as we’ve been watching “Game of Thrones.” Why do people in these fantasy / medieval worlds (e.g., “Lord of the Rings” and “Game of Thrones”) always speak with a British accent?

Martin has said English accents work best for fantasy, as the genre is rooted in the Middle Ages.

“It’s full of castles and lords and swords and knights and all the other trappings that we associate with England in this country. It seems natural. It would be hard to do with a group of actors who had thick Southern accents,” he has commented.

Hah, seems to make sense to me!