Ummmmmmm
Whereabouts are you wanting to hold this event and is it 'appropriate' ie The Basingstoke CND not welcome at Odiham.
Are you a member of the RAF Club for example.
HMS Belfast, Warrior and (I think) Victory can all be hired but these are at commercial rates as is SS Great Britain. RAF Museum, Fleet Air Arm Museum also good but expensive.
TA Centres can sometimes be used subject to the necessary vetting etc.
If you have any mates still 'In', would they sponsor/front up your dinner?
Is there a Service charity 'component' to the event - which can sometimes help
If you have a specific location in mind, write to top man, explain what you want and ask if he will give permission. Problems are; insurance, security issues, costs (no subsidised rates anymore), level of service can be variable. Most likely no accommodation overnight on base.
I organise a dinner every year which usually involves a visit to a military base before/after. I use civilian hotels and set down careful requirements which I want the hotel to follow and have learned the importance of close liaison with the hotel throughout (main need is to convince the hotel that you are not just a gash 'wedding party' but somebody for whom standards are important).
Hope this helps - I am available as a consultant and my fees are .........!!
Old Duffer