I don't think that MK. II's were proofed for nitro powders, but some older Webleys had later Mk. V style cylinders fitted by the govt. armorers.
I'd handload this pretty soft, but original ballistics should be obtainable; the .455 isn't a high velocity cartridge, anyway.
Nice find, and thank you SO much for the nice photos.
I'll pass this on to friends on a forum dedicated to a TV show where many of the heroes used Webley revolvers. Some will probably come here and look.
T-Star