If you visit http://www.productsandservices.bt.com/consumerProducts/displayTopic.do?topicId=28760 and click on the phone tab, you will be able to see that BT 'adjusted' their prices. What this effectively means is that BT put up (or down in certain cases) their prices. According to Section 48 of their Ts&Cs you can claim that the change resulted in you being materially disadvantaged and you can cancel your contract free of charge.
Spoke to the chairman's office at BT today and they confirmed. Remember you have 10 days from the day you learn about the changes to cancel your contract.