Many men left their civilian jobs to join the forces, but some in essential roles were not allowed to switch. What term was used for work like this, for example, coal-mining or train driving?

The Great War - World War One - was the first war in which Britain had been involved to see a significant 'Home Front'.  Civilians became caught up in the war whether they liked it or not, and life became very different during the war itself - and for some time afterwards.