Virgin only fly out of Glasgow. We STRUGGLE to get Glasgow flights. In fact I only managed it this time because I checked the Disney site every day until it showed available and booked immediately. Had a hassle on the webist and had to phone them and there were only 2 economy seats left on our return trip! That was 17 months out!
What are your dates? Are there no Glasgow Virgin flights scheduled for the period you want to travel? If you book direct with Virgin, they only open up their flights 11 months prior.
In the past I have opted to fly to Manchester and change there. I know it is a hassle, but with a little one on my own the thought of being in a foreign airport with a different way of doing things (and payphones that reduce me to tears) was just a bit much for me. I don't know how you are travelling or who you are going with. Also, the Manchester 10 ish departure gets you to Orlando at around 2 pm which gives you plenty time to get to the hotel, swim, rest and go out for dinner.
I wish Virgin would add some extra Glasgow direct flights though. These ones are always sold out way ahead of the time.