The United Kingdom is probably where Diplomacy got its start in Europe, centered around ManorCon and MidCon - ManorCon has been running since 1982.
Information about ManorCon can be found at www.manorcon.org.uk.
Traditionally, Diplomacy Tournaments in the UK have been part of larger Board Games conventions. Information about such conventions in the UK can be found at www.boardgamers.org.uk.