I am sorry you're leaving Virgin Media. We do bill in advance but I understand the confusion relating the final statement.
You say you've given notice to leave Virgin on the 2nd September, which means 30 days notice, so the service disconnects on the 2nd October. This means the billing time you'll owe for is up to the 2nd of October. Now let's say your billing period is the 15th-14th, and on the 15th September we have billed you. This billing period would be the 15th September-14th October. You'd pay that bill and then a couple of days after disconnection, get a final statement, for the days you've already paid for but the service has been disconnected. Ie a credit from the period 3rd October - 14th.