The British postal system runs on a system of alphanumeric codes, or Postcodes. Each postcode begins with a prefix consisting of one or two letters. Normally this prefix denotes the main sorting office through which the post for that place is filtered before being distributed to individual addresses; the only exception to this rule is with postcodes in the inner London area (but this does not correspond completely to other definitions of inner London).
Note that the postcodes, and also the full postal addresses, do not follow local government or traditional county boundaries. The London postal district is much smaller than the boundaries of the Greater London Authority and many areas in outer London have the more standard form of postcode.
postal counties are no longer required by Royal Mail; and if a postcode is supplied, will be ignored. Many people will still quote their address using them, however.
British Postcode Prefixes (including overseas possessions)