I don't mean to be rude here but dropping bullets into the throat and trying to seat them with the cartridge case just invites disaster - too easy to collapse a case neck, undersize bullet can get ahead of the case and so forth - any one or combinations of problems can turn that old Brit into a bomb.
Despite the massive amounts of INCORRECT information floating around the internet about black powder - you most certainly can blow-up a modern gun with black powder. Create the right conditions, such as an obstructed bore when the bullet gets ahead of the powder charge, and black powder can generate pressures in excess of 75,000psi.
For a total of $50 (shipping included) you can purchase a small Lee reloading press and a set of reloading dies and properly reload the cases and for half that cost you can get the little self-contained hand reloading kit
Prep the cases as normal for reloading clean, inspect, size, check length, de-prime & re-prime.
The .303brit is an easy BN case to reload but it still requires care and attention. You must ensure you have sufficient case-fill to prevent any possibility of an air-gap opening between the bullet & powder. I load for maximum consistency and accuracy using a drop tube and special compression dies I made and while you need to load carefully and consistently, you need not take it to extremes.
The Lee dippers are handy for measuring powder, consistency is the word of the day, do it the same way every time. Find the one that fills the case to a point where the powder is up into the neck no more than half the neck length. Gently tap the rim of the case on the table until the powder settles completely, it should not go below the neck - if it goes down below the neck, dump it out and start over with the next larger scoop. What you want is to have the condition where when the powder will not settle any further, it is just slightly into the neck, roughly 0.040" above where the shoulder starts. This condition then allow a very slight compression when the bullet is properly seated.
As for the bullets, you want to use soft cast bullets, sometimes WW alloy that has not been quenched will work fine but in most cases you'll find that cast bullets made from 10 pounds of pure lead and 1 pound of WW alloy will perform better. Bullets should be sized 0.001" to 0.003" larger than the groove diameter of the barrel you're running them through - measure the actual groove diameter of the barrel, don't just rely on mfg spec's because they vary for all guns.
Lube the bullets with a suitable black powder lubricant same as you would for a muzzle loader. Seat the bullet into the case. I won't get into the maximum accuracy issues related to adjusting the neck sizing and thinning necks or any of that - use a softer bullet alloy and they'll obtrude to fill the bore.
Word of caution: When you run the first loads, you MUST check the bore after every shot to ensure that the bullet alloy you are using is not too-soft and is allowing the bullet to shear lead off leaving it in the bore. This can be a dangerous condition as the lead left behind from the previous bullet can act as an obstruction!!!
Certain loads, powders, contaminants in the bore and environmental conditions can cause excessive fouling - excessive fouling will not only destroy accuracy but will also create erratic and excessive pressures - keep the bore clean!
I will not use any kind of filler or wad in a BN case - it just invites the possibility of problems and "possible problems" translates to "possible catastrophic failure of the gun". I don't know about you but I prefer when things do not blow-up in my face.
For powder, I would likely use 3F as the first choice - the larger the granulation of the powder, the more problematic it's going to be finding the correct volumetric measure and compression to prevent air gaps from forming because of settling.
