SilverWook said:

There was plenty of time to pick up a British accent between ROTS and ANH. I knew someone who moved down south after living most of their life in California, and they picked up quite a twang after 15 years. I didn't even recognize their voice on the phone at first.

Things just sound more evil when you're British. Imagine Tarkin or Palpatine with an American accent, and it just doesn't work. ;)

By British I guess you mean English but a posh Edinburgh accent could have worked with Tarkin as could a posh Bostonian accent.

Back when Americans were getting really good television from us on Masterpiece Theater and the like there must have been a feeling that we had actors (coming from a still thriving theatrical background with more gravitas) which was exploited along with the weak pound when filming.

If you look at Game Of Thrones you have almost every kind of British, American and Continental European, Middle Eastern accent on display there.

Some real, some 'acted' and the characters are all rather mixed (Charles Dance plays a rounded character which does nasty things but is also admirable in some ways).

There was clearly an attempt to make the English accents represent authority (either from the Old Republic end with Mon Mothma, Obi-Wan or from the Imperial end).

And remember not only is it an English accent it's an RP English accent which explains why Leia puts that on when she is being authoritarian.

Anakin should have been dark haired and develop the same accent as Obi-Wan.

In my opinion he should have been a middle player in the story (like Mace was).

That way the story could have focussed more on a character we know little about and know nothing about his fate.

Someone who rocks the boat, someone charming and interesting like Han was.

Someone American.