My favorite load for 215 grain bullets in the 303 British is 43.0 grains of IMR4350 over CCI 200 LR primers. If you use magnum primers drop the powder charge by two grains.
Velocities run right around 2300fps.
This is an old load developed by P.O. Ackley. It's very likely in his book "Handbook for Shooters and Reloaders"
Ken Waters also recommends this load as well as one other with powders available today. 46.0 grains of H4831 with standard large rifle primers for 2100fps and 46.0 grains of IMR4831 which is slightly faster burning for 2160fps, over standard large rifle primers.
Ken Waters did something unusual with his load work ups for the 303 British and the 215 grain cartridge in that he listed both the "H" and "IMR" versions of 4831 powder.
Back in the day, the writers of this information did their own pressure testing, usually by trial and error, rather than on scientific equipment, which back then was a pure copper pin, which was screwed down onto a piston, which had a gas vent from just in front of the chamber, which caused the pin to be crushed when the cartridge was fired. Then they measured how much "crush" there was on the pin to be able to extrapolate the resulting measurement into pounds per sq inch of pressure.
Ackley had the pressure testing equipment in his shop. Whether or not he bothered to check the results or even knew how to calculate the results, I don't know.
I've used all of the above loads and they all work well in the No1 MkI, No1 MkII, and No4 rifles as well as the Pattern 14 Enfields.
I've shot a few of these out of the Mk5 carbines and the recoil is brutal to say the least.