I own two Mk VI revolvers. The first one I purchased is an Enfield Mk VI (mfd. 1924). It's cylinder was shaved for 45 acp. It has New Zealand ownership markings on it in addition to the British proofmarks. It's in excellent shape. I've fired it once and now it just gets pulled out of the safe to show to people.
A few months ago I purchased an actual Webley Mk VI (mfd. 1917) with ownership markings indicating that it was owned by the British Goverment. It's not in as excellent shape as the Enfield Mk VI, but it's good. I got that one after a gun collector passed away. I and a few others were allowed to look over what was left of his collection before it was sold to a gunshop. I got the Webley for a fair price.
According to
Cartridges of the World 10th Edition, by Frank Barnes and edited by Stan Skinner the Fiocchi 455 Webley factory load is a 262 grain lead bullet with a an average velocity of 850 feet per second and a muzzle energy of 420. The old Colt factory load (pre-WW One) used a 265 grain lead bullet with an average velocity of 757 feet per second and a muzzle energy of 337.
I believe Enfield manufactured the Mk VI from 1923 - 1927 for goverment use. It was discontinued with the introduction of the smaller Enfield .380 Pistol No. 2 Mark 1 in 38/200. Which is a heavier loaded version of the .38 Smith & Wesson. I don't know if Webley continued to manufacture the Mk VI for commerical use after Enfield took over the goverment manufacturing.
I've talked to other Mk VI owners who download their 45 acp loads to 455 Webley standards. They seem to be happy. Personally I just dry fire the old girls with Snap Caps. Snap Cap makes a 455 Webley and you can order it directly from the company via their website. The 45 acp Snap Caps work just fine with half moon clips.
For the record I have put six rounds of the Hornady 455 Webley load through my Webley Mk VI. Very easy shooting load. I have put six rounds of a downloaded 45 acp load through my Enfield Mk VI. That's it. They are my safe queens. I've got plenty of newer shooters. The Mk VI wheelies are going to live a life of ease. My choice.
The Mk VI is a terrific revolver. Though the design is obsolete I treasure the two I own. The Mk VI harkens to an era that probably never really exsisted, but should have. Great revolvers.