Why is it BAe always seem to think they're the only people doing military flight testing?. Westlands in Yeovil (Somerset), Britten-Norman at Bembridge (Isle of Wight), International Test Pilot School at Coventry Airport, Flight Refuelling Aviation at Bournemouth Airport (Dorset) all are active in the field of military flight test (albeit none of them exclusively).
Other players are the Empire Test Pilots School at Boscombe Down (Wiltshire), Cranfield College of Aeronautics (Bedfordshire), CAA Flight Department at Gatwick (Sussex), Slingsby (North Yorkshire) and half a dozen microlight manufacturers. All have flight test departments of various sizes.
If you do as XFTE recommends and phone or write to their personnel or departments, there's a chance you might get something. You'll have earned it if you do just for having the balls to do so.
G