The Nov 2022 Home Office Guide on Firearms Licensing Law says:
12.88 Whilst there are no limits imposed on the number of solid projectiles that a
certificate holder may purchase for manufacturing ammunition, a home loader must
not manufacture more than the maximum quantity of ammunition authorised by
their firearm certificate.
12.89 Some reloaders may test ammunition by firing groups of 3 to 5 rounds, and
repeat the group several times to measure consistency. As bullet designs,
cases, primers and propellants are so varied, it is not unusual for home loaders
to require larger amounts of missiles (bullets) for their research, and possess
them in addition to any factory or other home loaded ammunition already held