That's pretty common: movie villains often tend to have British accents. Also, Lucas used a lot of British actors (most likely for the cost!).
Also, you've got a point: Britain still somewhat clings to the tradition of being an imperial power and America is supposed to stand for liberty and democracy in defiance of empire...*ahem* *cough*
I think pretty much all the Imperial officers were Brits. So too was David Prowse (the guy in Darth Vader's suit) and the Emperor himself (Ian MacDiarmaid).
Of course, so too were Alec Guinness as Obi Wan and Anthony Daniels as C3P0.
The latter is the other stereotype that British accents are cast with: the prissy, effeminate sissys.
We are all in the gutter, but some of us are looking at the stars.
~ Oscar Wilde