Nice, but this thread began as you asking about the MkVI. No need or worry about smokeless conversion for the MkVI.
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Lyman Cast Bullet Handbook, 4th Edition (2010)
p279 shows 231 in the .45 Auto Rim with #452374 225-grain LRN No. 2 alloy at OAL 1.265":
start 4.0 grains 231 for 660 fps @ 6,900 cup, max. 5.5 grains 873 fps @ 13,800 cup
(I would not exceed 5 grains in the Webley)
I would not be worried about trying the 4 grain charge with a heavier lead bullet up to 265 grains.
Any pressure-tested .45 Auto Rim or .45 ACP data you come across from a current, reliable industry source, which is below 13,000 cup is fine in your Webley.