Hi Caroline,
I can only tell you of my experiences last November and 4 weeks ago!
We did the Virgin web check-in (24 hours before the flight) and then went to Gatwick Airport for the Twilight Check-In. At the Twilight Check-In desk someone from Immigration (I Guess) puts a sticker on your passport cover to confirm that it has been checked. You are then able to proceed to the check in desk, where your printed boarding card is checked, luggage is weighed and has the labels attached. This is then deemed as Check-in completed. You do not need to return to any desks the following morning, just make your way through airport security.
I hope this helps.
Paul