Here’s an interesting piece of research, done in 2010 by one David Thomas as part of a degree in British First World War Studies:
This is a well-footnoted 38 pages, covering the British procurement of handguns, the different types of handguns use, and the training methods used with them. To me, the most interesting part was those training methods – I had not realized British training was nearly as elaborate as it really was.
The influence of [Captain Charles] Tracy is as the bringer of pistol fighting technique. In 1915 he wrote a simple instructional handbook for more advanced training, drawing attention to his skills and knowledge. He advocated the use of instinctive shooting at close ranges and using the sights for longer distances. Tracy’s stated “War Shot” standard was the ability of a man to hit a 12” by 16” rectangle, with one pistol shot at ten yards, in one second. He is acknowledged as the writer of the 1916 Addendum to Musketry Regulations, which improved pistol training enormously.
…Issued with Army Orders as a separate 15 page close-typed document, it was intended to be enclosed with Musketry Regulations. It included a much more advanced and demanding set of practices and tests than before the war.
These cover grouping, rapid and snap shooting, at a variety of ranges, and engaging targets whilst advancing on foot. Shooting is to be conducted with both right and left hand, single and double action, to tight timings. One practice involves firing at targets whilst moving down a trench. There are mounted practices for those for whom they were appropriate.
The Boer War had involved relatively little use of handguns, and their training and procurement lapsed between the end of that conflict and the beginning of World War I. At that point, though, it seems that the British really put in serious work to improve handgun shooting skills with their troops. Handguns were used extensively by officers of course, but also by naval troops, aviators, tank crewmen, and machine gun crewmen. As the author says,
From a somewhat simple and unsuitable procedure at the start of the War, British pistol technique developed along parallel but complimentary official and private lines, to a sophisticated level. Safe use, allied to skilled tactical methods produced first class combat shots. Anyone passing out from a revolver instructional course in the latter half of the War would have been better trained with a pistol than all but special forces personnel in the modern army.
Definitely worth a read!