The .455 Eley cases were 0.87" long, the same as the pre-1900 .455 Mk1 black powder rounds. The New Service revolvers were chambered for the longer Eley or Mk1 cartridges so that they could also use the older service rounds and commercial ammunition which used the longer case. The .455 Mk2 case is 0.77" long and works fine in the longer Eley or Mk1 chamber, when soft bullets of adequate .454-.456" diameter are used.
Many New Service .455 revolvers were rechambered to accept .45 Colt ammunition, which works OK, but reduces collector value. As for the Webley top-breaks many had the rear of the cylinder faced off to provide moon clip clearance for .45 ACP ammunition.
While the .45 ACP conversion works just fine in the Colt New Service, which is a robust, solid frame gun, in top-break Webleys in which the cylinders have often been "shaved" to accept .45 ACP ammunition with moon clips, it is dangerous to fire full-charge .45 ACP GI ammunition. This is because M1911 Ball exceeds the proof pressure for the .455.
Older Webley "marks" prior to the Mk4 Boer War model do not have heat treated cylinders and were not even deemed safe with .455 Cordite ammunition unless cylinders were replaced with heat treated ones and the guns re-proved during Factory Through Repair, for WW1 service.
Much reload data for the .455 is based on the longer CIL-Eley-Canuck 0.87" length cases and is dangerous to assemble and use in the shorter, more common 0.77" Mk2 cases as produced by Fiocchi, Hornady or Starline.
Safe charges in Mk2 cases with 255-265 grain soft lead bullets of .454-.456 diameter are 3.5 grains of Bullseye or TiteGroup or 4 grains of HP38, 231, or WST, or 4.5 grains of Unique at minimum cartridge OAL 1.18" with common .45 Colt factory .454" diameter 255-grain lead conicals or similar common cast bullets such as Lyman #454190. My design for the .455 is the Accurate 45-264D.