Yes, you can use .455 Mark VIZ in your Webley Mark VI revolver. It is nice you have one of these good old revolvers in its original caliber, which I am sure you have verified (?). Hundreds of these revolvers entered the U.S. market at a time when there was little ammo available for them - basically only Dominion Brand commercial ammunition, expensive for the time. Gunsmiths started shaving the back of the cylinder off to accept the .45 ACP cartridge in half-moon clips. This worked o.k. and made saleable (average price when these came in was, as I recall, between ten and twenty bucks, hardly worth the time and effort for a dealer to do the paper work to sell them, at least in a state like California). However, the conversion made them unsafe to shoot with the original caliber due to the huge increase in headspace caused by the conversion.
Today, one in the original caliber and nice condition (of course, they did not generally bother with the conversion of poorer-condition Webleys as it was not economically feasible, the price of the conversion being as much as a poor condition Webley was worth) is hard to find, and value higher than a new, good quality, commercial revolver.
As I recall, the Webley .455 Auto Pistol, which has its own rimless version of the cartridge, had a circular recess at the rear of the barrel so that they could be single-loaded with .455 Revolver ammo in an emergency, although the pistol became a single-shot, as you could not use the revolver round through the magazine. At Least, the military Mark I and Mark IN (essentially the same pistol) had this, as I recall.