Get on the phone and call them up. I did a few years ago. They can only say no.I wanted to get an old pathfinder pilot over from Australia a good look around. They were delighted to see him and gave us a full tour of the aeroplane. Needless to say he told them a thing or two. I don't think me being in the Senior Service gave me any advantage over any one else who asks for a visit on behalf of old crew. (Apart from calling them up on the mod phone network in the first place.)
Many, many. (about 20) years ago I also got a good look around by just turning up, via the public entrance. By chance we got talking to the adjutant, who on hearing all three of us were airline pilots, took us "over the rope" and gave us the full tour there and then. Think he thought we would give him a job. But as I said earlier. The best way to go about these things is head on. Ask the people that matter. Not all of us with nothing better to do on line.
(ref Mombassa, its now a couple of Ball Point pens and a disposable lighter. And if she was realy good a hotel soap as well. )