CZ_Brno summed it up well.
Magnum primers burn longer and hotter than regular primers. They will work for regular (non-magnum) loads but aren't ideal. An accurate load developed with magnum primers may burn different and give different accuracy with regular primers.
Some people say using magnum primers with regular loads can cause thermonuclear explosions killing everyone in a 5km radius. It's just a piece of old misinformation. They could cause overpressure but only in loads that would be near or at maximum pressure with regular primers (like CZ_Brno said.)
As for .303 British loads my favourite powder is H4895 simply because of it's universality. I use it for my .308, .303, 7.62x54R, .30-30, and my .45-70.
For .303 British I use:
150gr SP over 38gr of H4895
174gr FMJBT over 36gr of H4895
180gr SP over 36gr of H4895
90gr cast .32 pistol SWC (WW, water dropped) over 20gr of H4895 (still testing these, not happy with the accuracy so far)
Last one is just for cheap range plinking. They cost me $0.13/each so I can shoot my Enfield all day without breaking the bank (or my shoulder.)