Afy,
You can buy as much brass as you wish.
You can buy as many non-expanding bullets as you wish.
For expanding bullets, you must show authority on your Firearms Certificate (FAC).
You must show your FAC to buy primers.
You do not need to show anything to buy propellant (smokeless powder).
You need a special license to buy black powder.
I should add that expanding bullets and primers and black powder must be purchased "face -to - face". Mail order is OK for all others.
I am not sure, but it is possible that a mail order company would be able to send you expanding bullets as the restriction only applies to people living in the UK. Just another one of our stupid laws!
Charles