My nomination for AmE to BrE WOTY is "Wonk" as in "Policy Wonk".
Google searches of pages from the UK show a number of examples, and Oxford and Cambridge dictionaries online both list the AmE sense of the word (the Oxford also has the British Naval slang sense).
The clincher for me though was to hear “(Policy) Wonk” used on BBC Radio 4 by Jane Garvey during the 12 November broadcast of “Woman’s Hour” in a segment where she was debating “who are the women who matter in UK politics?” with Allegra Stratton, the political editor of BBC Two’s “Newsnight”. If it's on "Woman's Hour", surely that's a sign it's moving out from the "Chattering Classes / West Wing fans" and into the mainstream?
1. A student who studies excessively; a grind.2. One who studies an issue or a topic thoroughly or excessively