As El Presidente said, he's been with them for 10 years and this is the first time that he's had an increase. My wife has been with them for 5 years and it's also the first time she has had an increase.
I am sure that they will have something in the small print of the contract to say that they can increase the prices... they would be sure to have covered themselves!
If you're not happy, then the only thing (other than cancelling your contract) would be to speak to ofcom
about it. If you are worried about not being able to get out of a long contract, then speak to them and they will be able to give you advice.