Are you looking to supply the MOD with equipment (i.e. PCs directly to the base itself), or are you looking to ship to private buyers who live on the base? If it's the latter, I believe it's just normal postage rules. If it's the former, you probably won't get any luck. MOD has existing contracts with hardware suppliers for IT which prevents them from going to other sources.
As one of the few people in MOD with a vested interest in purchasing "official use", gaming-spec PCs, it's a pain in the arrse. Dell are crap at providing anything other than office desktops (they refused to source me an nVidia GTX) and they cost a fortune. I found out today I need a new HDD for the last PC they gave me (500GB raided was never going to be enough...) and I just know that a couple of terabytes is going to cost about £500 after their mark-up.
One other thing to be aware of, even if there is a route to getting on the "official suppliers" list: MOD doesn't trade with any company who can't show accounts going back three years. If you're a new company, you'll need to wait a while.
Well that one I couldn't answer - I am but a mere, UK-based civilian! I'd imagine that it's just a matter of saying that you can post to BFPOs (apart from Afghanistan... maybe) so people know it's an option. Might be worth looking into the tax implications as well, because you may get more business this way if you can offer tax-free prices.