The fact that you are "affronted" as oppose to "offended", that you want to enforce your cultures beliefs on another's literature, and that you claim to be British says to me that the choice of accent they chose for the Imperials was spot on.
Wait! Let's start a letter writing campaign complaining about the portrayal of Americans in Downton Abbey! No, really, let's not.
Seriously, though. If you play the Imperials, you will see that they think of themselves as the good guys. They are not one-dimensional evil bastards. They just have a different government structure.
Affronted is not the same as offended, affronted is offended by the whole concept, it's a broader offense, a cultural broadside if you will. Besides we are allowed multiple words for the same thing, it would be a boring language if we all had to comply to american usage, or american's had to observe english usage.
On the subject of the thread, as a British person I am doubly offended, firstly that the Empire seems to be based on the English, and secondly that it's a mock up of southern accents, where's the Brummies and the Lancashire/Yorkshire accents, and could the have at least one scouser in Nar Shadaa, after all we get everywhere.