From the Wikipedia page 
The Metropolitan Police Service (MPS) is the territorial police force responsible for Greater London, excluding the "square mile" of the City of London which is the responsibility of the City of London Police. The MPS also has significant national responsibilities such as co-ordinating and leading on counter-terrorism matters and protection of the British Royal Family and senior figures of Her Majesty's Government.
A number of informal names and abbreviations exists for the Metropolitan Police Service, such as "the Met", "Met Pol", "MP" and "the MPS". In colloquial London (or Cockney) slang, it is referred to as the "Old Bill". In statutes it is referred to in the lower case as the "metropolitan police force" or the "metropolitan police", without the appendage "service". The MPS is also referred to by the metonym Scotland Yard after the location of its original headquarters in and around Great Scotland Yard, Whitehall. The current headquarters of the MPS is New Scotland Yard.