I lived close to that area for 18 years, and drove past it twice a day for 11 years, before and after the M25 was built, and I've never heard of or seen anything there.
I had the relevant Action Stations at the time, and the book covering all military airfields, so if there had been anything mentioned I'd have gone to look.
Broxbourne Aerodrome is gravel pits and has been for many years. There's absolutely no trace of it now. It's hard to believe there could ever have been an airfield there.