Star Wars villains in the first series use such accents.
A) The Imperial Navy is a euphimism for the British Navy / Empire.
B) It was filmed in the UK with almost all UK actors.
The new Star Wars - none of the accents are posh really.
Just English accents, again actors, filming etc. - although - I think there was some attempt by JJ to keep it UK-ish in that manner, though I'm not sure.