My other grandad wasn't allowed to join up because he was a supervisor at the Steelworks factory where he lived and it was deemed more important that someone who knew what they were doing stayed behind and made sure the right metals were being made for ammunition.
I had an uncle who was a Royal Marine Commando between wars but apparently he did some pretty nasty things that he wouldn't talk about (which might have done him some good, to be honest).
Another uncle was in the RAF until a few years ago. I think he was serving at the time of the Falklands war but I'm not sure.
Karen