If you wanted to 'sell on' the broadband then you would also need to keep the BT line in your name and be responsible for the bills. If you cancel your phone service, the broadband gets cancelled too.
If you are on BT equipment at the exchange there is a setup cost payable to BT, that is why they like to lock you in, they have to recoup that one off cost. There are broadband providers who do month-month but you get hit a setup fee for that, I think. Apparently the setting up of broadband in the exchange requires a couple of men to go in with a tool kit to make a physical connection, it's not automatic in the existing generation hardware in most exchanges.