If I remember rightly from the BALPA conference Virgin's requirements were 3500hrs just for them to even bother reading your CV. And those 3500hrs preferably on large jet with EFIS. Military pilots with that number of hours would be considered though.
Chief Pilot at Virgin came accross as a top bloke and kind off stole the show at the end with his witty and upbeat presentation.
With 300 hrs you might just be lucky enough to get a FO job with a small regional flying turboprops earning significantly less then your present RAF salary and you have to go flying regardless of the weather generally.
If I were you mate I would stick with your nice salary, housing provided, no debt military life style. I know all about both worlds having been in the dark blue uniform for 6 years. Get as many hours from the RAF as you can and as you near the time to leave push for some big jet/multi crew time on the Tristar or Galaxy.
Then and only then, will Virgin be interested in you.