That's a good question. The answer usually given is that after their experience in WWI, the British decided that a lighter gun was better, and the best way to get that was to go to a smaller caliber with a heavy bullet. But why they chose an obsolescent under-powered cartridge dating from the black powder era, when much better cartridges (like the .38 Special) were readily available, remains a mystery.

One must remember that like most European armies, the British considered revolvers as secondary weapons, and were more of a badge of rank for officers than anything else. Therefore, caliber was relatively unimportant.