Hi Lisa,
I would call VA on 0870 190 4403 and just explain the situation.
The process is normally:
Call above number and they ask your date of birth, if you can swim and a few basic details about education levels.
If they are happy with that they will send you an application form.
They are really nice to speak to so I would call them and just ask if they will send you an application pack now as its not long until you are 19. If they say they will send you an application pack then ask them if you should return it now or once you have reached 19.
Good luck and if you don't get into Virgin I have heard BMI are short of crew so fingers x'ed it won't be too long b4 you get a training date.
You will love flying I've done charter work mainly and really enjoyed it. At the moment I'm working for Qantas London base and its not so good. In fact I'm applying to Virgin myself.