"I would do terrible things to get a WW2 era .455 Webley." Well, there were no .455's made in that era; the standard caliber was .380 by that time. The .455 Mk VI was still being issued, though.
Folks wonder about the scarcity of surplus ammo for the .455 and .380. The answer is that there was not much made. Neither cartridge was used in anything else (as the 9mm was used in the STEN) so little was needed. The standard issue was one (1) 12-round box to each person issued a revolver, with one (1) more box in unit supply. That was it. The soldier got his 12 rounds and was told to get on with the war. (In American forces, an officer or soldier armed with a .45 pistol could often draw as much ammo as he could shoot, for practice.)
Another note, if you are fortunate enough to own one of those .455's cut for use with .45 ACP in moon clips, please don't fire it with factory or GI rounds, not even .45 Auto Rim factory loads. Even the Mk VI has been known to blow with .45 ACP GI loads, whose pressures far exceed the proof load for .455. Stick to light handloads, no hotter than low-end .45 Colt standard loads. For Mk's I-V, black powder may be best!
There is a myth that Webleys are super strong and cannot be blown up or damaged by overloading. It is a myth. And has been "busted", along with guns.
Webley certainly produced a variety of guns. There is a joke among Webley collectors that the company once made two guns alike, but they corrected the error and never repeated it.
Jim