I e-mailed Virgin about this and this is their reply:
Thank you for your e-mail. Please accept my apologies for the delayed response.
Yes, you can use Twilight Check In and drop your bags off the day before your flight. Just let our check in agent know you used DIY check in and they will print your boarding card and issue you with bag tags.
Here's some further information regarding our Twilight Check In service:
* Passengers can check in for their next day flight from Gatwick the evening before
* Continental codeshare passengers are permitted to use this facility
* Twilight Check-In is open from 1200 - 2100 daily, at the
Virgin Atlantic desks, in Zone A of Gatwick's South Terminal
* As usual with check in, you'll need your passport/s, ticket/s and any luggage that is not being taken into the cabin
* To use it, all party members must be present with the required documents
* You'll be issued with your boarding card which you'll need to bring along with your passport for the flight
* Passengers must leave an address and telephone contact number with the check-in staff in case there is a need to contact them overnight
* The following day, passengers need to be at their assigned gate no later than 50 minutes prior to departure
* If any passengers decide to check in more luggage on the day of the flight, they will need to queue up in the normal way
* Passengers who want to check in either surfboards or windsurfers with the rest of their baggage, will not be able to use Twilight Check-In. This is because we do not have a secure place to store them overnight
* Some bulkhead seats may be available, though many will be held back for the day of departure. Any passengers assigned bulkhead seats are advised that if on the day of departure there is a medical or infant /bassinet requirement, they may be repositioned.
We look forward to welcoming you onboard.
Kind regards
Syreeta Moulding
Contact Centre Customer Service Team