Ben Yagoda has a Lingua Franca post on an often-discussed phenomenon, “why, in American movies and TV shows set in foreign or imagined lands, the characters almost invariably speak in British accents, especially if they’re bad guys”:
The invaluable website TV Tropes dubs the custom “the Queen’s Latin” and has this explanation for its use in historical dramas:
Britain’s long history causes British accents to seem somehow “older” — they are used to suggest a sense of antiquity. This is actually inaccurate from a linguistic perspective; the modern British accents actually represent a more evolved form of English. Older English accents were closer to modern Irish and American accents.
In any case, using the Queen’s Latin makes a series or film commercially viable in the U.S. It alleviates the need for subtitles, while maintaining the appearance of historical authenticity. It’s just foreign and exotic enough. (Many British actors already Play Great Ethnics.) It’s also no doubt inspired by productions of Shakespeare‘s plays set in Ancient Rome. Remember: Romeo might have been Italian, but he’s not realistic unless he talks like a proper British toff.
(That last link mentions “the exaggerated smack of a boxing glove” and notes: “Real-life fistfights tend to be eerily silent, which obviously wouldn’t be very dramatic or exciting.” I never knew that.) And it’s not just movies and TV; Yagoda discusses a book that “is set in France and Germany during World War II, yet the author, Anthony Doerr — an American — continually uses British terms: crisps instead of potato chips, lift instead of elevator, and biscuits.” The sun may have set on the Empire, but this silly tradition shows no sign of going away.