Out of Context: The original trilogy was filmed in England, so most of the extras (and even many of the leads) ended up with that (set of) accent(s).
In Context: Different parts of the galaxy have different accents; Corellians don't talk the same way Alderaanians do, frex. What we hear as a British accent (remember that we're seeing the films "translated" into English from Basic), is actually the accent of the Core Worlds. Since the Imperial military is an aristocracy, most of the officers come from that region, and the ones that don't will fake the accent.
"Don't f_ck with the Jedi Master, son." --Mark Hamill
[This message has been edited by Flying Beastie (edited October 02, 2001).]