If you are still in your 30 day cancellation period before leaving, VM charges you as per normal (and collects payment via DD on the usual schedule) right up until your date of disconnection. Once you are disconnected you get a final bill with either a credit or debit due. If in debit you pay that off and leave. If in credit VM should refund you the balance.
Cancelling your DD is never a good idea with VM. All it will do is set in motion VM's debt recovery processes (which is a part of the company that does work very efficiently). The end result of that process can be missed payment charges, a default on your credit record and contact from debt collectors. There are lots of topics on here where customers have been badly affected by this.
Regarding all the problems (and since you have now cancelled) you will probably have to go to arbitration to resolve.
A required first step is a formal complaint to VM
https://www.virginmedia.com/help/complaints
but this will, most likely, not result in any useful outcome.
If VM fails to provide a satisfactory resolution, you can reject the resolution and VM should provide you with a deadlock letter (although OFCOM is currently investigating VM because of poor performance in doing this). With a deadlock letter, or in any event after 8 weeks from when you formally complained, you can go to arbitration.
https://www.commsombudsman.org/our-process
