I've found that to do the Paid offers under earn points > trials and paid, the ones in which your cell phone is involved you do not actually need to pay the full price. Heres a tip if you are on Pay as you Go
1. If you top up monthly or something just let you credit run low. (preferably under £1 to be sure)
2. Visit the offer and complete the requirements (such as an IQ test which seems to be a popular one)
3. They will ask you to submit your mobile number
4. Do that
5. When they send you a text, have enough credit left to reply "Yes"
6. Then they will reply and so on and so forth
7. Mark as complete if you have finished what it on the webpage (pressed continue for example)
8. Then you can text "stop" to the same text to stop your subscription
By doing this, they cannot charge you the sucscription fee because you do not have enough credit obviously. But the offer will still approve.
Make sure you have enough credit to send at least two texts if you are using your regular SIM!!
You could also use another technique of ordering a free SIM card from Tesco (UK) which has a very small amount of credit on, just enough for you to reply "yes".
Hope this is helpful and enjoy reaping the benifits