London Lost Property - The Guide
Tips on where to go in London to reclaim your lost items.
http://www.timeshare.freeserve.co.uk/tourism/lon/lost.html

London Taxi
Advice on how to recover property left in a black cab.
http://www.london-taxi.co.uk/taxi/lost-property.htm

Talking Cities
Contains a short note on where to go to retrieve your lost property in London, especially at the main airports.
http://www.talkingcities.co.uk/london_pages/essentials_jl.htm

Transport For London
Tells you everything you need to know in order to reclaim or hand-in property lost on the London Transport network.
http://www.transportforlondon.gov.uk/tfl/ph_lost.shtml