So by that logic isn't it more likely that Lucas created the Empire based on the Nazi's then anything else? Afterall they are the most known and feared in these days...
Of course he based it on more then just that. But the nazi's probably had a pretty big inlfueince in his story because it was the most known at that time, the one that still lived in the minds and hearts of the people and the one with most media coverag, so the message could be brought across a lot easier.
You try making a movie about an evil empire where people dress like people dressed in American. Where the army's have uniformas like the normal American uniforms with somethign resembling the American flag on it. And then to top it all have them destory a peaceful society.
Like said before the voices are only british because the movie was made in England... I doubt it had any bigger meaning, and that Lucas wanted it to go like that. Maybe he liked it the way it turned out, but i doubt it was the reason for what happened.
Oh and Darth isn't dutch for dark... Darth has no meaning in dutch at all.