Well thats right, they "have" to provide you with the MAC address, they "have" to let you go too, but then "dont have" to stop charging you as the contract is in place.
Just because they are not paying BT for the line once the MAC address has been leased, doesnt mean they dont have costs. In the first place, most isps make a loss, they have fees to pay to initiate the line, equipement costs, etc, and they "do" take people to court if they are going to make a "loss" If the case was 7 months, then it could be feasable that they have broke even, but contractually, the contract is binding and can be enforced, it wout be the isps choice.
BT are introducing charges soon to people who cancel there broadband and dont transfer to another ISP within the BT exchange. These fess come into force around July, they have been delayed as ofcom dont "agree" with them, well more the costs, this was due to be forced May 1st but its been prosponed.
You could well sell on the broadband and phone to the next customer, you would need to talk to BT on this one about transfering the agreement, or you could just have an agreement that the new tenants pay you.
I have always made sure that the landlord has the internet and phone in his name, this way, after my 9 months is up, i am not tied into paying him as its his responsibilty, its worked for me...