Found a nice but altered Webley Mark VI last week. A lot of these were ground or shaved down by importers back in the day, to use .45acp ammo, which was more readily available in the 50s and 60s. Even now shootable supplies of .455 Webley can be scarce. Fiocchi among others load it but its not always available.
Anyways the important thing to remember is factory .45acp is way to hot for these guns. At anywhere between 19,000 and 21,000 PSI, it is like shooting a proof load in those old revolvers. The Webley is rated for ammo creating pressures around 13,000 PSI. Being that these are now approaching a century in age it is wiser to load it softer than that. You can use and shoot .45acp safely, if you use downloaded ammo. I had a buddy load some 222 grain lead bullets for me. Velocity was 715 FPS with a pressure of 9,800 PSI.
You can see the results on this old pizza box, shooting it rapid fire at 15 yards. Actually put two through the same hole though the group is nothing to brag about, it certainly would have all been on a man sized target. Not too bad for a 101 year old revolver using ammo it was not chambered for.