You can obtain tickets to many venues in London with a 2 for 1 pass, located on any of the English train sites. You pay full price for one entry and get the second one free. Google it and you will find the deal--they say you need a train pass in order to use the 2 for 1 passes. I printed the passes out before leaving the US, and since there were 6 of us, I printed out 3 of every possible thing I thought we might want to do. Since we were staying outside of London and taking the train in every day, I knew we qualified. But the interesting thing--when going into a venue (i.e., Tower of London, St. Paul's Cathedral, etc.), I would present the passes, get the 3 free tickets and NO ONE, over 3 days, every asked to see the train ticket that qualified us to use the passes. Since the tower was the equivalent of $25 and the Cathedral was $18, and the passes cost nothing to print out on your home (or hotel) computer, it's a no brainer!