This thread is quite old, but I was just wondering the same, so I jum into it… Basically, especially in ESB, the British accent wors as a class-accent for stiff elite Imperial officers, and also it gives a flavour of “Empire and colonies” feel. I think that works well, and we are so used to have Brits accents for Roman soldiers (Masada, I Claudius, The Caesars, Rome), or just many Germans in old WW2 movies (speaking either with a very British accent like in “Inside the 3rd Reich” or with terrible fake German ones). Anyway, for good or bad reasons, this high class snobbish accent is easily associated with scheming militaristic generals ! My 2 cents…