My Father, a skilled electrical engineer, volunteered, was signed up and waiting for a travel warrant for his posting as an officer in the Royal Marines, when he got an essential war worker designation served on him and spent the rest of the war just outside London on 'secret' work as a civilian. I suppose it is OK now to say that he was developing tiny engines to operate ejector seats in RAF planes. The general public and the 'enemy' were not aware that our aircrew had such things.
His elder brother remained in a reserved occupation in the engineering works where all 3 brothers were working (all same job) at the start of the war.
Youngest brother however, was conscripted as soon as he turned 18 and sent from our coalmining area to Tyneside to replace miners who had joined up (He survived but many of these amateur miners died down the mines.)
Meanwhile a former colleague of mine in the Bank was conscripted and became a gunner in the far east. He informally swopped duties with his unit's Pay Clerk - an innumerate, former miner from South Shields!!!!
The reality during both WWs was that POLICY & PRACTICE were poles apart, the War Office just muddled through and censorship kept the public in the dark.