Seeing as it seems I am not getting an invitation (humph!) how about Peter Hart. He is Head of Oral History at the Imperial War Museum, a very enthusiastic historian and I am sure that he would be a great A-D speaker.
You have to watch out for him though, if he finds out you have served, in any capacity, he tries to get you to tell your story to a microphone!
It might be best to ask the speaker how long he/she would like to speak for; an experienced speaker, such as Eric Grove, will have spoken at such events many times and will be able to tell you in advance how long he would like for talking on a specific topic.
The intention is that it will be a not for profit dinner. Any monies made over and above the cost will be donated to charity. Probably RN/RM. I'm looking into doing a charity auction. A major sporting venue is in the running for hosting the dinner so signed merchandise may not be out of the question.
Just to reiterate that I am using these suggestions as part of my proposal to get permission to crack on so nothing is Definate as yet.