Having flown at Kenley many years ago, as I recall the airfield is only active when the VGS are flying at the weekends.
Don't know what number you've got, but if you google Kenley's VGS you might well find a mobile number for the OC. I suspect that if you were talking about going in while they were active, they might well see the benefit for the cadets to have a guided tour around a shiney helicopter...
That said, I'm pretty sure the site is owned by the local council - so it might all be too tricky.
Best of luck...