My understanding was, that until the advent of WW2, from the days of the spread of the Empire, through the first World War, BritishOfficers were all "Gentlemen" members of the "Upper Class." They were expected to provide their own sidearms. During WW2, while some still purchased their own side arms, they could be issued a Webley MK IV .38/200 or No 1 Enfield in .38/200.
Webley and Enfield produced sufficient revolvers the their colonial wars, the enormity of World War One overwhelmed British production capabilities. The American company Remington, produced P14 Enfield in .303 for the British. While it was the British government that requested Colt and Smith & Wesson produce revolvers for the war effort in .455 Webley, I do not know the exact process that got the American made .455s into the hands of the officers.