Yep, they'll be the same account, no way to limit it as far as I know.
Is this a new Kindle? The current batch do have whispernet access for buying books in the UK (and elsewhere).
"The test of any good fiction is that you should care something for the characters; the good to succeed, the bad to fail. The trouble with most fiction is that you want them all to land in hell together, as quickly as possible."
— Mark Twain