<shrug> an antique revolver with shotshells is a good thing to have if you have rats in your barn... but it's definitely uncharted territory. There was data published in the Handloader Magazine in July of 1979, looked it up just now... I would not use their data as is for the Webley - it is meant for a 1911...
avr-developers DOT com/45shotshell/hl80.html
I've made my own shotshells _from scratch_ to use in a webley, but I am absolutely not comfortable sharing my data or suggesting it's good to try - there were a lot of changes throughout the time Webley revolvers were made, some may be weakened by the use of full load 45acp etc etc. I also used a mk6 to start the experiment before trying with a more expensive antique...