You can cancel, by letter, to this address. This gives VM 30 days to close the account and send a final bill. Send the letter "signed for", keep the proof of posting, and when the letter is delivered there will be a record on P.O. tracking website which will tell you when it was delivered, and who signed for it.
Virgin Media Sales Operation Support
Diamond Plaza
Daleside Road
Nottingham
Nottinghamshire
NG2 3GG