It depends on your pistol's throat and bore measurements. Softer cast bullets will perform better in in a lot of the webleys because the soft lead bullet base will bump up to grip the rifling better than hard cast bullets can.
There is no good reason to shoot hard bullets in a webley. Hard cast bullets won't hurt it, probably, but it really depends on how much you shoot. The steel in the MKI/II is softer than modern barrel steel. I've looked at 6-8 MKI/II pistols with the rifling pretty well shot out of them. I don't think it's coincidence that all of these had been converted to .45acp and shot with hardball ammo.