I'm not sure about the nationality being a distinctive mark if it comes to the quality of anybodies writings.
It's not, really, it's just a mark of style. Depending on one's upbringing, culture, and environment their writing is usually styled in a different way. Additionally, they tend to write about different ideals - there are much more American books about westward expansion and the wild frontier than British books, for example.
Aaaanyway, I'm part British =D I know a lot of Americans are, I mean my grandmother is actually from London. That doesn't mean anything, but it's true. I know quite a bit of British authors. I'll exclude the others mentioned, though:
There's, of course, Brian Jacques (the Redwall series), Mary Shelley, Virginia Woolf, Oscar Wilde, A. A. Milne, Phillip Pullman, Bram Stoker, Lewis Carroll, Runyard Kipling, R. L. Stevenson, and on and on. It's hard trying to remember them off the top of my head and reeeeeally hard not to cheat and look since I'm wikipedia-ing them to double check and make sure they're british.
On a semi-related note, T.S. Eliot is technically an American author but, like you, wanted to be British and became a citizen over there.
