The Collaborative International Dictionary of English v.0.48:

Bobby \Bob"by\, n.
   A nickname for a British policeman; -- from Sir Robert Peel,
   who remodeled the police force. See Peeler. [Slang, Eng.]
   --Dickens.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bobby
    n 1: an informal term for a British policeman

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "bobby":
   Dogberry, John Law, bluebottle, bluecoat, bull, constable, cop,
   copper, dick, flatfoot, flattie, gendarme, gumshoe, officer, paddy,
   peeler, pig, shamus, the cops, the fuzz, the law