At the time of writing, Wikipedia introduces its article thus:
Perfidious Albion is a pejorative phrase used within the context of international relations and diplomacy to refer to alleged acts of diplomatic sleights, duplicity, treachery and hence infidelity (with respect to perceived promises made to or alliances formed with other nation states) by monarchs or governments of the UK (or England prior to 1707) in their pursuit of self-interest.