I have used
http://www.topcashback.co.uk for years and last time I looked I had about £150 waiting to be paid to me. It will be left to accumalate until Xmas.
Some amazing cash back if you are thinking about changing insurers or ISPs... have a look at what you can get today only by switching to TalkTalk.
I thought everyone used it .. the other one to register with is ..
http://www.quidco.com/ .. they both pay out without any issues.
Every time I buy anything I just type the suppliers name in the search box in either of the two I have shown and more often than not cashback is available.