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.
I went to a Transformers convention earlier this year and met Michael Bell (I also almost adopted a cat he was trying to get rid of, but that’s another story), which is where I learned that he was the voice of the rebel that greets Leia when she arrives at the Yavin base. So that’s how I discovered about that small detail.