Really with all due respect what tosh,that's such a wild generalisation it's not true,the main difference is that Lucas has went for experienced British actors for specific older parts,to a large extent he's went for unknowns for the younger parts who are mainly American it's hardly fair to compare McGregor with the young guy who plays Anakin,who does a decent job but not much more I admit.
Logically actually this makes sense since McGregor has got to try to become Sir Alec Guiness,he needs a bit of weight there and obviously he's went mainly for lesser known actors for the younger parts so not to detract from the characters-also Anakin has only got to get the black suit on later,not become Jimmy Stewart.
As for your Harry Potter analogy that's off too since the author Ms Rowling (she's Scottish)only allowed the book to be filmed if they stayed true to the story which is set in England and for British actors to play the part,since she is one of the UK's richest women-she didn't need the cash from the movie-quite simply if the studio didn't agree the movie would never have been made.
She certainly didn't want her movie Americanised but that wasn't to do with the quality of American actors rather she wanted the movie to stay very close to the book,most authors even the extremely rich ones do not usually fight to keep their vision uncompromised.
I agree to some extent that historically(British actors had the stage tradition a lot of Americans didn't) that may have been the case but surely nowadays the acting coaching,experience etc. must be closer....