IMR4320 is a good powder and meters really accurately from most measures due to the short stick shape.
It is quite dense as well,for it's size although in the .303- that is not important.

My old Pacific loading manual (Pacific Tool Company, Lincoln Nebraska) lists loads for the .303 when in P-14's , which tend to be slightly higher than those normally listed for the more springy Lee-type Enfield rifles.

If you wish these loads, please PM me and I'll give you the list. They run all the applicable IMR and most Hodgdon powders.

Interestingly enough, although not surprising, BLC2 is THE powder for 150gr. bullets for this ctg. in strong actions - 2,780fps with a starting load at 2,600fps.