As Simon says Quidco are the best at 4%, I prefer Rpoints then claim thru their Highest cashback policy, 100% claim rate in 3 years where I had problems with Quidco. There is a thread right now on another forum with 1 or 2 peoples Quidco/Virgins claims not being tracked.
Use this site to check who pays best cashback.
http://www.moneysavingexpert.com/shopping/cashback-sites-comparison/
There is nothing on Quidco's site to say you can't get cashback for Tickets so its worth a try.