Revolution in Britain. The Empire refuses to go along. The United Kingdom is soon based in Ottawa, or Canberra, or Delhi. (Or with an early enough POD, Philadelphia.)
If it's a bunch of scattered colonies under one crown, especially before the evolution of dominion status, the name "United Kingdom" wouldn't be too much of a stretch. The empire holding together from a new capital even though the old one's long lost.
(Now, it wouldn't be the United Kingdom of Great Britain (or Great Britain and Ireland) mind you.)