Mandy, Virgin's Downtown Disney check-in is a useful feature which was stopped after the terrorist attacks of 9/11. It now looks like it is re-opening, which is great news.
The facility (which is in the car park at Downtown Disney, near the AMC movie theater if I remember correctly) opens each day 8.30am to 12.30pm, although it would be wise to check these times. You check-in in much the same way as you would at the airport: you need all your hold luggage, all your hand luggage and all the people you are travelling with. You will check-in your hold luggage, be issued with boarding cards and confirmed seats. The advantage of this is that you do not need to get to the airport until about 60 minutes before your departure time, rather than at least a couple of hours as you would normally. As the flights back to the UKi depart in the early evening you have a more or less full day to enjoy in the parks or somewhere else (although be aware that a heavy day's shopping is not advised as you will have already checked-in your luggage!).
Regards
Rob