I took a look at the MOD flight permissions through dji and surprisingly it lies outside the exclusion zone (just), possible to fly with just a responsibility warning, not even applying for permission.It's been pretty secure for ages, with alarms in most of the rooms, but it was recently sold and has a pending planning application going through which may explain why it's changed a bit.
Hopefully it gets sealed up and properly protected again before the goons fuck it over, it being where it is the MOD plod will likely take a dim view of some bellend flying a drone around or big groups of idiots bumbling about.
Easier to fly at than my local spot by heathrow!
That's not to say they wouldn't make the line of site claim or whatever