Maybe I'm being dense (I've worked 12 hours today and am beat) but I fail to see how Darth Vader is considered British. Just because the dude inside the suit is British doesn't mean a damned thing, especially when hardly anyone knows that fact (except true fans). The voice is American and there is no facial expression to go by...

The dude inside the Chewbacca suit was British, too but I don't see anyone complaining that Americans alway portray Brits as tall and hairy with limited verbal skills...