Normally if the fee is equal or greater than £240, you won't have to pay back any more than £240.
Cancelling and finding out your cancellation costs are something to call in about. Do that by calling 150 from a VM phone or 0345 454 1111 from any phone -> Option 1 -> Verify your details when prompted for them -> Option 4 -> Option 5. That option is staffed between 8AM to 8PM M-F and 8AM to 6PM Sa. They can tell you how much you have left to pay and put the 30 days notice through for you.
______ Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.