London doesn't have zip codes, we have post codes and they're far more specifc than zip codes, a postcode denotes either a single street, part of a street or in some cases a single building, In Greater London most codes start with a Geographical directions e.g NW for northwest followed by futher letters and numbers to narrow it down. On the outskirts of Greater London you may no longer get Geographical postcodes as the places were orginally outside London e.g I live in Chipping Barnet and my postcode starts EN, I used to live Edgware and the postcode started HA.
If you know the address or part of the address, there is a postcode finder at http://www.royalmail.com/find-a-postcode