More tourist passes by Dabs
If you are traveling beyond London, you might also check out the Great British Heritage Pass which includes a lot of the castles and houses outside of London (also Wales, Ireland and Scotland)http://www.visitbritain.com/world/heritagepass/
or the English Heritage Overseas Visitors pass which just includes sites in England
http://www.english-heritage.org.uk/default.asp
Or the National Trust pass