As everyone else has already mentioned, no it's not. Even if it was, it's still not a good option, as the bullet diameter is .452 on ACP, which is 3 thou smaller then .455, some of the bores on Webleys are even larger.
This will result in horrid stringing of shots, even key holing from as close as 10 feet. It's so bad that throwing the gun at the target would produce better results, I'm not kidding.
The solution? The poster in this thread "Jethunter" makes some damn good projectiles for these old girls. I was at the point in giving up on my Webley's, until I seen that someone makes good projectiles for them. Now it's shooting to point of aim, making solid hits on steel poppers as far as 50 yards. The investment into an antique firearm should also come with the investment into even a single stage press.