You need to ask BT to port your phone number across, which they will be able to do providing the line is active and you haven't asked VM to cancel the phone line. When BT port the number across that will cancel the VM phone line.
I suggest you first port the phone to BT before you look at phoning VM to cancel the TV package, and ask them to regrade your account to an internet only connection. This should avoid any issues with transferring the phone number.
The Service you do for others is the rent you pay for your room here on Earth - Muhammad Ali