The airfield used was RAF Langham on the north Norfolk coast. The 'Island of Children is supposed to be Scolt Head Island, but a lot of the filming was done at Blakeny Point, a few miles to the East. I think the fighters came from the Day Fighter Leader's School which I gather was at RAF West Raynham at the time.
Langham is now a Bernard Matthews turkey farm, but one of the taxiways was being used as a light aircraft strip when I once landed there in 1989 - according to my logbook. There used to a maintenace place specialising in De Havilland types. I've no idea if it's still there. There's a very rare dome shaped building surviving at Langham which was used as a training facility for Air Gunners.
A chap I used to be in a flying group with was an extra in the film as a child. He was living in Langham village at the time.