Perfidious Albion is a pejorative phrase used within the context of international relations and diplomacy to refer to acts of duplicity, treachery and hence infidelity (with respect to promises made to or alliances formed with other nation states) by monarchs or governments of Britain (or England) in their pursuit of self-interest and the requirements of realpolitik.
Perfidious signifies one who does not keep his faith or word (from the Latin word "perfidia"), while Albion is derived from an ancient Greek name for Great Britain.
Read more about Perfidious Albion: Origins and Use, Fascist Italy and Colonial Propaganda, Cultural References