If you completely terminate the service that is a cease and has been chargeable for several years now.
The charge is currently £24.74+VAT however it will raise to £31.12+VAT with effect from 5th May.
If you migrate the service to another provider with a MAC there is no termination charge - it is only chargeable if the service is completely terminated from the line.
Notice given of the cease has no baring on the charge. It arises because BT charge the ISP to terminate the service.