Prior to WWII
Bullets (and barrels) for the .45 Colt were nominally .454. After WWII, the size changed to .451/452.
Your Webley can be shot with .45 ACP bullets, I would recommend lead, and not quite so hard as bullets intended for the autoloader. But only if you are not getting acceptable accuracy from regular ammo (hard cast bullets). If you are, then it isn't worth the trouble to use softer lead alloys.
You might even want to invest in a single box of .455 ammo from Hornady. Some of the converted guns can still fire .455 safely. Many cannot. I bought one box, just to find out, and I did find out, that my Webley Mk VI can no longer safely shoot the .455 ammo.
Stick to GI ball equivalent handloads in your Webley, or lighter loads. Do not try to load it up. If you want more power, get a different gun.