Well you get your opportunities, just depends on the occasion.
I have been inside a E3 Sentry (B707), seen inside a Dominie, been inside a Tutor and Vigilant (with cadets of course) and got a photo of me on a ramp stood up close next to a red arrow... which I saw the inside of.
I think it was an F4 or something, but I have sat in both of its seats... though it was just the cockpit in a museum.
Ive been in two helicopters too... One on a flight around my neighbouring town, Nantwich
Vid here and this air rescue helicopter at RAF Waddington.
So yeh, with things like this you do get your opportunities... but thanks to a handful of idiots, we can no longer gain such access unless with very good reasons and probably a large wads worth of proof of identity.