Nivsy
You've had your answers: RAF Fairford is the nearest military base capable of handling the aircraft. It's in a relatively remote location. It has pretty good security and control of access is easier there than at Cardiff. Brize is very busy and probably doesn't need the hassle. Road links to Cardiff are pretty good, especially when you have lots of motorcycle outriders. Finally they won't be paying parking fees at Fairford. What more is there to discuss?