We are moving house, and when I asked to bring my account to the new place, I was offered slower service (understandable line restrictions) for more money. I did manage to find a better offer elsewhere - twice the speed for half the price, so when I declined the Plusnet got a bill of 150 GBP for cancellation. Why am I charged in this case - you cannot provide the speed I currently have, surely this is not the same service I have the contract signed for!?
You took out a contract for Plusnet to supply services to your old property. It was your choice to move house and in doing so break this contract. I gather that the cancellation charge would have been waived if you took the best service that Plusnet could provide through their network at your new location. It was only applied when you declined this offer.