The .45 ACP uses slightly higher pressure than .455 was designed for.
The 45 ACP uses almost half again as much pressure as the loads the 455 was designed for. I wouldn't call that "slightly higher". Plus the chambers and throat on the Webley weren't designed for jacketed bullets, which would raise the pressures still higher.
Read the link for more info.
I agree that they are too nice to risk blowing up. If you have an original, uncut Webley, shoot it with Fiocchi factory ammo
. If you have one that's been cut, shoot it with low pressure handloads with lead bullets.