Easy peasy, and much cheaper than buying individually

"The only gripe I have is that I didn't take the 5-in-1 package. Don't stress, this site is 100% legit and the tickets worked perfectly as advertised. You are emailed the tickets which you print and just take with you to each of the attractions. You need to book for ... Madame Tussaud's and it's not changeable, but the other attractions can be changed up to 10 times each. Note, none of the attractions validated the times, so if you have a 4pm ticket and go at 2pm you'll be fine - just say it's a 2pm ticket. May not work with Madame Tussaud's though as we were there on time. If you're in London for a few days, I highly recommend getting this ticket (or the 5-in-1 if you are there for 3 days or more)."