Back in the 60s the RAF had a number of other branches trained as aircrew. Remember in those days the RAF was flying at the edge of technology. Aircraft such as the PR9, V-bombers and Lightning were flying at extreme altitudes where the knowledge of human physiology was still limited.
It was essential to have properly qualified personnel to explore these boundaries. More recent boundaries of course were supersonic low altitude escape and also heat stress when wearing full AEAs.