Is this a MK V or VI with the thicker cylinder? If so, the Lee 2nd reloading manual has an excellent section for the 45 Auto rim, any load below 14000 cup is all right. IF an earlier MK, I would advise BP, or a load of no more than 4.5 grains Unique under a 230 lead bullet.
A handy thing to remember is that the .45 auto/rim has the same internal volume as the longer Webley MK 1 case, also known as the .455 Colt Eley, and data can be found on the internet. Also, if you have the Lee 1st Reloading manual around, it has a section on the .455 Webley (Mk 2 short case). Lots of good loads there for Unique, Bullseye, Green Dot, Red Dot.
You will hear folks tell you you need .454 or .455 bullets. The Webley has somewhat odd dimensions, with the barrels .451 +/- .001 BUT with the chamber mouths .449 +/- 001! This was an intentional restriction to help get a good pressure curve burn with the chopped cordite used way back then. Thus .451 bullets are fine, but they need to be soft! Ironicaly this does help get a good burn with Unique, which seems to have a burn rate close to that of chopped cordite. (Unchopped long strings of Cordite as used in the rifles burns a lot slower) No, no one seems to know why they call it a .455. Maybe to differentiate it from the .450 Adams round it descended from.