If staff can't get it sorted, then since VM have failed to allow you to cancel by providing an adequately staff telephone response, you can almost certainly get released from contract without penalty, refund of any payment they take (and possibly fifty quid compensation) by going down the formal complaint route asking for a deadlock letter if the company won't agree to that. If/when the complaint is fobbed off then you use the deadlock letter to take the matter up with the industry complaints adjudicator, CISAS.
So give the forum staff the chance to sort things amicably with a simple cancellation, that's quickest and best for all concerned. If that doesn't work, formal complaint in writing as per Virgin Media's Consumer Complaints Code of Practice. If that still doesn't work then escalate to CISAS and in that ask for fifty quid compo for the unnecessary hassle.