The reason for this is that, at the time (the late thirties) shirts were not actually part of the English policeman's uniform, but just an additional layer of clothing, considered as underwear, that the cop would add to his other clothes for his personal comfort and which would remain invisible underneath his heavy buttoned up tunic at all times. On cold days, the officer could keep himself warm by putting on not only a shirt and an undershirt, but sometimes a sweater and a scarf too!
The one cop who does not wear a shirt and thus finds himself in just his underwear, not roped up with his colleagues, but lying on the floor on his own is the police sergeant.
That part, as far as I can see, remains uncredited.
