With UKicon National Service you could "Defer" your "Call-UP" for certain reasons and under certain conditions. I'll see if I can reproduce the rules which are included in the notes in a larger scale pic another day. It was quite common to "Defer" Nation Service while studying at college etc and then do National Service or in my uncle's case, after "Deferring" and spending 4 years studying at college, National Service had ended.