Most medium or fast pistol powder can be made to work. Unique. Bullseye, Titegroup, IMR700X, IMR800X, etc. Unique is my favourite. When you are looking at internet load data, make sure you understand whether it is for the .455 webley MKII case or the 45acp case used in a shaved 455 webley pistol. Case capacity is different and loads are not 100% interchangeable between the two.
As Cyclone said - Soft lead bullets work better, whether hollow based or flat based. Mostly any soft bullet can be made to work. I use flat based bullets because generally the will give better accuracy, but nearly anything will work.
Generally: Size to .454 or .455 for the S&W hand ejector models; Size to .452" for the MKI and MKII webleys. Flat based bullets are just as accurate if not more so than hollow base bullets. The advantage if there is any of the HB bullet is that it increases case capacity.