You're thinking mainly of WW II, not WW I.
Pattern 1937 holsters were made in two models. One was for the Enfield No. 2 .38 and one for the remaining Webley MK VI .455's still in service. They varied in shape as well as in size.
The .38 holster was also used with Colt and S&W revolvers in WW II. And some were used with Colt .45 autos, which were used extensively by commando and paratroop units and by SOE and SAS. Churchill expressly wanted the Colt .45 auto, which he personally carried, for his newly formed commando forces. The ctg. also interchanges with their Thompson SMG's, which some retained after the 9mm Sten gun replaced it in most forces.
Tankers had an open-topped rig, with bullet loops for .38 cartridges sewn on the front. This originally had a long drop strap, for the gun to ride low on the leg. It was a nuisance that way, and many were modded to use a shorter strap. I think they were eventually made that way.
As this is an S&W board, I'll mention that Newsweek or Time once had a cover photo of King Hussein II of Jordan. He was wearing a Model 19 in one of those open canvas webbing holsters. Pretty nice wood in the grips, too.
The Mills company did make some webbing holsters during WW I. Those I've seen in photos were for Colt .45 autos, which were rather popular with officers, who bought their own pistols until 1915. After 1920, they had to carry the issued model, the Webley MK VI. However, the Ministry of Defence cheated Webley by not placing more orders for MK VI. Instead, they blatantly stole the design and made the guns at Enfield arsenal. You can tell a true Webley from the Enfield made MK VI by the markings.
Some Colt 1911's chambered for the .455 Eley AUTO ctg. were bought in WW I. I don't know which holsters they used. The RAF used these in WWII; issued them to Coastal Command.
Officers wearing Colt autos often had leather holsters with a spare magazine pouch on the side, as with German holsters for, say, the P-38, Browning 9mm, etc. but because the guns were private property, the holsters were of no set pattern and varied a little. I've seen one such holster for a .25 Webley, worn by some field marshal. He did not, of course, expect to see battle.