Well as said before, the truth is in the TOS (Terms of Service) for the ISP.
If you are on a 12, or even i have seen an 18 month contract, then you are "tied in". There is no escaping paying the service, as if you dont pay, they will see you in court, and with small claims court been so cheap and clear cut in a fair ammount of cases, they are doing this alot.
Companies do have to give MAC addresses out, they have to release you, but only if the contract is expired.
Its just the same with mobiles, if you stop the direct debit, the payments you owe will mount up, and eventually the collectors come in to play.
If you take our a Contract, its legally binding, BT or no one esle can do nothing unless the company who you took the contract out with is going to do nothing, or they let you go.
You can find way the isp has violated there agreement with you, this rendering the contract void.
In the case of "Clara" was it above, it would be feasable that if they did take the guy to court, there was no evidence, and they woudl loose, so it wouldnt have been in there interest.
Chaning lines could solve the problem of getting connected again, but remember, you still put you name to a contract, and in the UK and every other place i can think of, a contract is legally binding, and you are legally bound to fulfill that contract if the contract terms are maintained.
ISP's have an SLA too, thats a service level agreements, so if they are not providing you with the service they said, this could be grounds to render the contract void. Rather than turn you back, it be better to get them to cancel to agreement