I would also recommend limiting shooting to tailored handloads in the Webley top-breaks. I don't trust any commercial reloads or factory fodder for this.
Matt's Bullets offers a copy of the .455 service bullet, as well as a .45 HBWC that kinda/sorta approximates the .455
"Manstopper":
Both shoot well in my MkVI, which unfortunately also features a shaved cylinder. A case loaded with Trail Boss about 90% full (to the base of the seated bullet) gives good, consistent results at safe pressures. Decent accuracy at about 550-600 fps, depending on the bullet. They also shoot just fine in my other .45 ACP revolvers.
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I load my Webley fodder in .45 Auto Rim cases to segregate from the full-power .45 ACP loads for my 1911.
There's some .455 Webley reloading data here that can serve as a starting point. Reduce a bit -- .45 ACP cases typically have slightly less internal volume.
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Since Auto Rim brass is pretty scarce right now, consider some other method for distinguishing the lighter Webley loads from standard .45 ACP, assuming you shoot both. Perhaps storing loaded ammo on moonclips instead of in boxes?