First, fatality isn't the proper parameter to discuss; stopping is.  Let's not go down that rabbit hole!

Second, people survive handgun wounds about 85% of the time (don't ask me to source that statistic; I only recall it from my distant past).  So, again, killing people isn't their forte.   Obviously, handguns are used to stop crime much more than any other weapon – it must not be their lethality that counts. 

According to Gary Kleck, in something like 95% of defensive firearm uses, the gun isn't even fired.  All this tells me that the caliber of the gun, or more importantly, its stopping power, is secondary to the perception of the guy on the muzzle end!  In countries where handgun possession is severely restricted, the very presence of a gun, even a .380, is enough to put a damper on crime.  In our Old West and many big city neighborhoods today, the bigger, the better.  Still, this remains a matter of perception.  We want the biggest, hottest bullet we can comfortably handle, but that is partly for insurance and partly for peace of mind.

In that light, the Webley, with its big, punkin-roller bullets, is a viable defense weapon - if you don't mind its size, weight and aesthetic shortcomings.

Stu