I don’t really see what the issues are:
1) You have an FAC and permission to hold a firearm and appropriate ammunition. You have a few different deer legal cartridges on your ticket. You have bullets, primers, brass, powders and reloading dies appropriate for those cartridges.
You don’t have a firearm in 9x19mm, nor the authority to possess ammunition in this calibre. Yet you can still buy the component parts. But why on earth would you want to possess 9x19 brass, bullets, primers, powder and dies if you don’t have authority to possess such ammunition??
In the past as soon as soon as you made a 9x19 cartridge you would be in breech of the Firearms Act. Now, if the CPS can show that you “intended” to make ammo for which you have no authority then you would be in breech.
I suppose you go to the gunshop and you pick up powder and bullets for a colleague or friend. Well it will be down to your own judgement that he has a valid FAC. If you start buying components for “mate in the pub” then you may have a little explaining to do.