It is not a matter of splitting cases, it is a matter of pressures. The old .455 revolver cartridge had a working pressure of around 11,000 psi. The .45 ACP has a pressure of 21,000 psi, or almost twice as much. There have been Webley Mk VI's blown using .45 ACP, and the Mk VI is a stronger gun than your Mk V.
The Mk V, BTW, is a fairly rare gun, with only 20,000 being made in 1913-14. The conversion hurts the value, but even so, it is highly collectible.
If you shoot it, I strongly suggest you load the .45 ACP with .45 Colt data (NOT RUGER ONLY data), or use .45 Auto Rim cases with the same light loads.
Please do not accept the claims by a few Anglophiles that the Webleys are super strong, can't be blown up, etc. Not true.