"Except in the circumstances detailed in paragraph 51, if we let you know about a change to the price or the terms and conditions of a service which is to your material disadvantage, you will not have to pay a charge if you decide to end that service early, unless the Tariff Guide says otherwise. However, once we have told you about such a change, using one of the methods detailed in paragraph 50, you must let us know within 10 days that you want to end that service by giving us the written notice set out in paragraph 21. Only the services that are directly affected by the changes we make can be ended without paying an early termination charge for that service and this agreement will continue to apply to any other services that are not affected by those changes."
You have to wait untill they tell you that they are raising the prices - it will be with this month's bill. Then you have 10 days to write to them to cancel. You don't have to give them any specific notice, but you must write within 10 days.