If you fit primers that are NOT INERT, you would technically be creating ammunition to which Section 1 (and/or other sections depending on the projectile/s) of the Firearms Act 1968 would apply, and your newly primed collection would then be subject to licensing.
Are you also aware of the requirements of Section 35 of the VCR Act?
[i]It is an offence for a person to buy or to attempt to buy-
(a) a primer to which this section applies, or
(b) an empty cartridge case incorporating such a primer,
unless he falls within subsection (5).
(5) A person falls within this subsection if-
(a) he is a registered firearms dealer;
(b) he sells by way of any trade or business either primers or empty cartridge cases incorporating primers, or both;
© he holds a certificate authorising him to possess a firearm of a relevant kind;
(d) he holds a certificate authorising him to possess ammunition of a relevant kind;
(e) he is a person in the service of Her Majesty who is entitled under subsection (6) to acquire a primer to which this section applies;
(f) he is entitled, by virtue of the 1968 Act, the Firearms (Amendment) Act 1988 or any other enactment and otherwise than by virtue of being a person in the service of Her Majesty, to have possession, without a certificate, of a firearm of a relevant kind or of ammunition of a relevant kind;
(g) he is in possession of a certificate authorising another person to have possession of such a firearm, or of such ammunition, and has that other person’s authority to purchase the primer or empty cartridge case on his behalf; or
(h) he is authorised by regulations made by the Secretary of State to purchase primers or cartridge cases of the type in question.[/i]