I was there in March, GGG I believe is being kept for competition use for ammo supplied events. Picked up some Hornady A Max black 155 grain for club use, people who used it rate it highly, not cheap at £125 per 100.
I can confirm Durham have a unique way of interpreting the latest guidance which I understand is November 2022. They have allegedly told colleagues that the Law, not the guidance requires cameras for renewals to take place. As far as I can see the guidance does not mention cameras anywhere in there list of security requirements.
It is my understanding Alliant RL series of rifle powders are made in Europe and are compliant with latest H&S requirements, Lovex is also made in Europe. I have used RL15 in 308 Win for bullets up to 175 grains. This is priced in one pound containers, Vhit comes in one kilo containers and works out cheaper per cartridge, I now use Vhit N140 for this reason.
I used to load various birdshot and slug, now only reload three and a half inch cartridges, regular sizes are at a reasonable price, a RFD near me is selling 70mm seven and a half’s, fibre for £253 per thousand, can’t remember name as never seen this make before.
I would suggest finding which powders are available near you, lots of shortages everywhere, consult a reloading manual, I prefer Hornady although others are available. It may be prudent to attend a reloading course (NRA or other) these courses are subject to much discussion on this forum.
I was shooting a comp on Short Siberia at 200 yards, ten rounds in ten minutes was the course of fire then, I was use a 22 semi auto for a small bore comp, on the same firing point was a full bore rifle with a muzzle brake, I was asked by the ROs if I wanted to wait, I declined and fired my rounds off in between his with no problems.
They will be fine, Fed Match or CCI BR4 primers are preferred for 1000 yards or longer as this gets the best out of the small primer cases (usually involves higher pressures) see recent post about N150 powder and Laurie Hollands reply. His views are always worth listening to.