Details of London's primary Tourist Information Centre are given below. There are other more minor centres and those are listed in the relevant district articles.
Britain and London Visitor Centre (BLVC), 1 Regent St, SW1Y 4XT (nearest tube station Piccadilly Circus). M 9:30AM-6PM (Oct-Mar), M 9:30AM-6:30PM (Apr-Sep), Tu-F 9AM-6PM (Oct-Mar), Tu-F 9AM-6:30PM (Apr-Sep), Sa 10AM-4PM (Oct-May), Sa 9AM-5PM (Jun-Sep), Su 10AM-4PM, Public holidays: 10AM-4PM. Closed on 25 and 26 Dec and 1 Jan. Visit London is the official visitor organisation for the capital and has a lot of free information for visitors in several different languages. It also acts commercially and can have some astoundingly good last minute deals on accommodation.