However, as a result of the deteriorating international situation and the rise of Nazi Germany, Leslie Hore-Belisha, Secretary of State for War, persuaded the cabinet of Neville Chamberlain to introduce a limited form of conscription on 27 April 1939, with the Military Training Act being passed the following month.
Only single men 20 to 22 years old were liable to be called up (*), and they were to be known as "militiamen" to distinguish them from the regular army. To emphasise this distinction, each man was issued with a suit in addition to a uniform. The intention was for the first intake to undergo six months of basic training before being discharged into an active reserve
(***). They would then be recalled for short training periods and attend an annual camp.
At the outbreak of war, on 3 September 1939, the Military Training Act was overtaken by the National Service (Armed Forces) Act, and the first intake was absorbed into the army. This act imposed a liability to conscription of all men 18 to 41 years old.Britain did not completely demobilise in 1945, as conscription continued after the war. Those already in the armed forces were given a release class determined by length of service and age.
In practice, releases began in June 1945, and the last of the wartime conscripts had been released by 1949.
en.wikipedia.org/wiki/Conscription_in_th...dom#Second_World_War
(*) which explains why my father was 20 at the time !
(***) Which probably explains why he did 18 months of his service abroad !