Varget and BLC2 will do everything you need with either of those cartridges. BLC2 actually got its teeth cut on the 303Brit as BLC dureing WW2.
BLC2 can be temperature sensetive and Varget doesn't seem to be bothered as much, so it would probably be a very good place to start for botr cartridges.
I've put several newbies onto this in the past and many just stick with the load they develop with them that gives the best results in their firearms. They are both very resiliant powders that cover quite a spectrum of case capacities and give outstanding performance in most.
I know I don't need to caution you on this, but please be extremely careful in load development at this time of year. No matter what powder you decide on, a safe load at -25 can be a grenade at +30.
Another bit of advice, a decent scale (not neccessarily expensive) is your best buddy at this stage of the game and is the cheapest insurance you can buy.
Ask for some manufacturers handloading pamphlets. It will save you a bit of money. The only drawback is they don't have instructions on useing the equipment.
The 303, should have the bore slugged before you buy bullets for it. Bores can and do range anywhere from .308 to .318. If it is on the .318 side, there is an 8mm bullet available that will fit nicely. I don't know what is available at Epps these days. Back in the days when Elwood was around ( I really miss that man) they used to carry every size from .311 to 315, made by CIL and Kynoch. Elwood was the one that put me onto the .318 bullets designed for the original 8x57 mauser.
Bearhunter