I seem to recall from the 'CESR' briefs at Cranwell that you salute more Senior Officers (obviously) of all services, members of the Royal Family, Senior Officers of Other nations' armed forces, Cabinet Ministers and anyone in a Coffin. They also mentioned ladies too but it was briefed that that has tended to fall out of favour.
I've probably missed a few there such as Flags, ensigns, bits of boats etc.
I seem to remember the Gp Capt in 'The Battle of Britain' Film salutes a Jnr Officer's wife in one scene.