Last weekend my O2 account expired. Thanks to Apple O2 were forced to unlock my phone which meant I was able to switch to another provider.
After looking around I found a good PAYG deal from T-Mobile. I applied for a free SIM which arrived in two days. Then I topped-up £10 to get free texts and internet in the following month. Out of that top-up I purchased a £5 internet booster that gives me free internet this month. A booster is available for £20 which gives internet for 6 months.
So, depending on my usage this month, I could have free internet for two months for a £5.
Compare that to £35 per month.
If you mainly use data and can switch I can recommend T-Mobile in the UK.