Something I never understood about British manpower deficits in WW2. They raised an army of millions in World War I, so why couldn't they repeat the feat in WW2 with a (slightly) bigger population? Instead, it seems they had difficulties raising army units from day 1. Is it because the RAF sucked in so much material, resources, and manpower?
There was a far greater requirement for men to be retained in industry during WW2 given the far greater use of heavy equipment and aircraft etc that was supplying the Imperial forces plus allied units compared to WW1 and this partially reinforced by lessons learned in WW1 when the rush to volunteer had a detremental impact on some of those vital industries ability to support the then war effort.
If you were a skilled worker (known as a reserved occupation) that was deemed vital for the war effort then you could neither volunteer nor be called up.
In the end so great was the demand for coal miners for example that a significant % of those called up became coal miners! Grey cells are telling me this was 10%!
I was once informed (???) that one of the reasons why the commando units were so good was because they were partially made up of men who came from reserved occupations as the rules against them volunteering did not extend to the Commandos.
Also as mentioned the RAF was a vast orginisation - a Bomber Squadron with its ground support 'slice' of personnel had nearly as many men as an Infantry battalions personnel 'Slice' of personnel.
And while there were RAAF, RSAAF, RNZAF Squadrons under the RAF Umbreller while efforts were made to have their personnel 'slice' from that nation - in practice it very rarely got to 50%
Then there was the navy - again far larger than in WW1 what with many hundreds of escort vessels and the personel necesary to support them plus the Merchant navy
Then there was the 'British' men that were in the Indian Army units - many (not all) of the Indian infantry Brigades had 1 British Battalion of the their 3 Infantry Battalions many of the Indian Army units officers and Specialists would have been British as well.
And this would have been the same for many of the other 'non dominion' Commonwealth divisions such as the 2 West African Divisions.