More than likely Civilian pilots who work at Filton.
And If you have permission from Air traffic, (Its not impossible) game on
A couple of years ago, there was a witch hunt for a guy who recklessly flew low level into RAF Brize Nortons airspace and foolishly buzzed the tower. Bringing the full wrath of the the MOD and the CAA upon us all.
That was until it transpired he had permission, so that's OK then.