So it would be right of you to expect 30 days notice too have been given once letter was 'signed for'
Unfortunately due to issues with Virgin's various department currently its very hard to give exact advice on what happens, but you have done you part and would go ahead and remove direct debits now that the let is recorded as delivered.
You may be able to see if any actions have been taken on your account by signing into the My Virgin Media portal and checking if any orders are pending such as the example below. see here for orders. https://my.virginmedia.com/trackorder/view-orders