Why do the Sith all speak with a British accent and the Republic all have Yank accents? What is that supposed to mean? And they are not even real British accents but Americanized ideas of them... Was there a reason for that?
Because Tarkin had a British accent in Episode 4. And seeing as how he was played by Peter Cushing...Oh, and some the people on Alderaan have British accents.
Though simply it's just another line between the Empire and the Republic/Rebels in the SW universe.