Grandad and great grandad served in wwi and wwii need help?

Basically i am trying to find if there is a way to access names of who joined up in the army so i can find out who my great grandparents where and if they had more that two children and as for my grandad i've just found out he was in the airforce and am nosey and wanna see what it says about him unfortunetly i read it has to be next of kin who requests info on my grandad for mod but grandads next of kin was my gran she died in 1961 and my dad is alive but we are tight arses us lot and wont fork out for things haha so is there any way of getting this info free?