I would be very surprised if there was an opening for a PPL to fly on an AEF.
There is generally a waiting list to join and several hoops to jump through before you are approved to fly with cadets.
The sort of flying that the AEF's do is not just cruising around with a cadet admiring the view. Most want to do aerobatics so a PPL would have to be qualified to do some fairly complex aeros - you would be surprised what cadets want to do!!!
The AEF's offer some of the most rewarding flying for ex service pilots as most cadets are keen and a pleasure to fly with so places are sort after.
Beagles point about flying a G- reg a/c doesn't really apply as they are flown under military rules and you don't need a civilian license.
HF