The Collaborative International Dictionary of English v.0.48:

Britishism \Britishism\ n.
   1. an expression that is limited to English as spoken by
      Englishmen (especially as contrasted with American
      English).

   Syn: Anglicism, Briticism.
        [WordNet 1.5]

   2. a custom that is peculiar to England or its citizens.

   Syn: Anglicism.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Britishism
    n 1: an expression that is used in Great Britain (especially as
         contrasted with American English) [syn: Anglicism,
         Briticism, Britishism]
    2: a custom that is peculiar to England or its citizens [syn:
       Anglicism, Britishism]