it's gonna cost you just to walk away. a mate of mine signed up with Bt Internet on the basis he could have several routable IP addresses and a 20:1 contention ratio, he got 1 IP address and a 50:1 contention, connection dropping out every day, eventually ended up going to a lawyer, ended up having to stay with them for a year 'cos he couldn't afford the thousands it would have cost to sue BT.
surprisingly enough, my problem was with BTInternet as well shocking
I managed to leave my contract early on the grounds that THEY broke the contract....paid my solicitor to call up customer services and tell them that I want out. They released me straight away...it wasn't a major surprise to them really, I had complained about it every day for almost three months. Sueing for compensation is not an option though with a Telecom company like BT....they're just too damn big.