There is a camp site set up right by Fairford, with a bridge access to the airfield usually. The website should point you in the right direction for this.
Take plenty of water with you and, depending on what you plan to do, a large coolbox. Food isn't cheap and neither is drink.Best place to sit for photographs is either end of the runway.
Just go and enjoy, but get there early.
Cheers
Skiddy