That's what my friend had the Parker Hale No4 Deluxe Supreme. With a Nikko Sterling 'scope on that P-Hale mount that replaced the standard No4 backsight.
When he went shooting in Russia in the 1970s, bears, they took him to GUM in Moscow as he needed more ammunition. He thought that it might be a problem. No! When he asked for it the man simply said "Yes sir, 174 grains or 215 grains, what do you prefer?" and pulled out boxes and boxes of the yellow and red Kynoch stuff. He siad he saw more there on the counter than he could ever have hoped to see in one place in England!
Don't be too disappointed about the Rigby 303 Mausers the one I saw, and was at the time friendly with its owner, shooting it at Bisley had a very worn bore that was, when built, quite generous in its dimensions as really only worked well with oversized bespoke (or that awful word "custom") swaged bullets. Or cast lead bullets. I can't remember which was over twenty-five years ago. But with standard 174 grain Mk VII ball it shot like a shot gun and couldn't hit the side of a barn even when standing inside the barn.
In fact I think that the rifling, from memory, was of Metford form. Which of course had eroded quite badly. But will stand to be corrected.