OK firstly the deal: You can get 'buy one get one free' on most London Attractions! The attractions include:
City Cruises boat trips
Chicago theatre tickets
Tower bridge exhibition
Tower of London
ZSL London Zoo
... and many many more!
Just vist the website and print off a free voucher!
All you have to do, is travel by train to London. However, you can also use Day Travelcards (paper tickets not oyster) as a valid ticket, or family travelcards, cheap day returns, saver returns, etickets, advance purchase tickets, season tickets.. almost any type of rail ticket is valid. (see: http://www.daysoutguide.co.uk/faq.aspx
for more info)! If you are not travelling in by train (i.e. live in the west end/city, or using buses) just go to a staffed station and buy a ticket for a rail journey to a London Terminus.. i.e. Cheap day single from Acton M/L to London Paddington for about £2, just to get the offer!
So here is an example, two people are visiting London for the day, from Clapham Junction. They each get a cheap day return to London Waterloo for £3.20 each, they plan to go to Madame Tussauds, normal door ticket price for an adult is £24.. so you get BOGOF tickets when you present your voucher and rail tickets, so save £24... easy !!
Go for a day out this summer to the capital, visit some of the worlds most well known attractions, and best of all.. SAVE SAVE SAVE !!!