If you have a .455 Webley that has been shaved, the solution is easy. Get yourself some .45 Auto Rim cases from Starline.
If I had never had a "shaved" Webley, I would recommend this, too. However, since I do have personal experience I have to tell you...
THIS DOES NOT ALWAYS WORK!!!!
I have a Webley Mk VI (Mark 6) dated 1917. It has a "barely" shaved cylinder. LESS then most. .45Auto Rim brass WILL NOT WORK, the rim is TOO THICK. Also, it will not work with any full moon clip I have found.
Again, they are too thick, for my gun. It does work fine with S&W HALF MOON clips. (the thin metal 3 round clips).
When the British surplussed the Webleys, they also surplussed ammo. Both the guns and the ammo were sold in the US from the 50s, on.
When the supply of surplus ammo ran out, no more was forthcoming. US makers didn't make any, either. Various distributors and others then "shaved" the cylinders of many Webleys, to allow the use of .45ACP BRASS in clips. The idea was to allow the guns to continue to be used, using commonly available .45acp brass to load Webley level loads in.
Different people did the "shaving" and different guns were shaved in slightly different amounts.
However, the fact that one should NOT USE .45acp AMMO in a Webley was not well communicated, and since .45acp ammo will fit, and fire, many people simply just used it. And many Webleys were destroyed as a result.
Standard .45ACP ammo is loaded to what is, for a Webley, proof level pressure. .455 Webleys are big, they
look strong, but they aren't!!
Shooting proof loads through the guns eventually results in them failing. Some well before others. No doubt you can find someone who has been "shootin .45auto through my Webley for 50 years, and no problems!"
And that is true. However, that Webley might go another 50 years with no problems, or it might fail on the very next time it fires a .45acp round!!! (cracked cylinders seem to be the most common failure). There is no way to know.
My gun is barely shaved, and it will fire .455 ammo (I got a box from Hornady some years ago, don't know if they still carry it), and fired 6 rounds. TWO suffered pierced primers. SO I don't shoot .455 in my gun, I shoot .45acp CASES with Webley level loads.
Different guns, shaved a slightly different amount can work fine with ACP brass in clips, or Auto Rim brass, and not fire .455 with any reliability.
My gun won't run with AR brass, rims are too thick. The gun will close, but the cylinder will not turn if I use AR brass.
I have never found a full moon clip that would even allow the gun to close when it was used. And, I quit looking, some time ago. Half moons (S&W, not other brands) do work, and so do some 2rnd clips I got somewhere.
SO, while USUALLY .45AR brass will work in a shaved Webley, its not an "always works" thing. It depends on how much your individual gun was cut. Shaving the cylinders was not done to a uniform standard, and variations do occur.