The Days Out Guide provide some great 2 for 1 deals in London. All you have to do is print a 2 for 1 voucher, buy the ticket to the attraction at the site (as opposed to online), and enjoy your savings. But be sure to read the fine print. In order to retrieve the 2 for 1 voucher, you are supposed to show that you have used some kind of public transportation in London.
During the Olympics, there were 2 for 1 deals with the Oyster card, which is used for bulk travel on the Underground. All you had to do was print a 2 for 1 voucher and show your Oyster card when you purchased the tickets for the attraction, and voila, you received two tickets for the price of one! One attraction we used a 2 for 1 voucher for was the London Eye.
We heard that the lines for the Eye could be dreadful so we showed up when it opened at 10am. It was a beautiful clear day, and surprisingly, the lines weren’t so bad. The total experience took about 50 minutes with 30 minutes on the actual Eye itself (so 10 minutes for the line to buy tickets and 10 minutes for the line to get on the Eye).