It seems like Lucas overdubbed basically anybody who was British (except Alec Guinness) in the first film. Though somehow General Tagge survived. Maybe because his working-class East End voice didn’t sound like a stereotypical upper-class Brit.

Probably it was Irvin Kershner on ESB who decided to let the Imperials use their natural British accents, for the American Revolution symbolism. Lucas, by contrast, thought of the Empire as more like modern-day America, so most likely he wasn’t the one who came up with that.