Cancelling the direct debit will provide plenty of work for the debt collection industry, just what we need at the moment as unemployment rises and people are looking for jobs.
If you do as you intend VM will treat non-payment as breach of contract, which it certainly is, and will default your account, add late payment fees and pass your contract to VM collections who will, in due course, pass it to the debt collectors for action. In addition a six year addition to your credit record as a defaulter will scupper any future chances you ever had of further credit.
VM lines are busy at the moment, but you can through if you try, and early in the morning when they open is best. 8am weekdays and 9am tomorrow.
But for cancellations you can write:
https://community.virginmedia.com/t5/Managing-Your-Account-Cable/cancellation/m-p/4593541#M147624
That thread will help.
As for forgetting your telephone memorable word, it is most likely to be your mother's maiden name. That's the first question on the list. However you don't need it. After getting through don't enter any characters at the prompt. The phone system should redirect you to an agent for alternative security clearance.
Now I have no real interest in what you do, but don't say ever that you haven't received clear and unambiguous advice here on the consequences of taking such ill-advised action.
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