This may help, copy/paste from the "Home Office guide to Firearms law" Nov 2022 edition;

Occasional firing

8.14 An antique firearm can only be held as a curiosity or ornament and cannot be fired.
However, they can be added to a firearm or shot gun certificate for the purposes of
collection and occasional firing. Where the ‘good reason’ for possession is
collection and not target shooting, section 44 of the Firearms (Amendment) Act
1997 requiring membership of a club to be named on the certificate is not
applicable. Where a person has an antique firearm which they wish to fire for test,
research, re-enactment, target shooting or competition purposes, no test of
frequency of use should be applied when assessing good reason to possess: the
primary reason for possession will be collection.

8.15 An antique firearm may be brought on to a certificate or removed from time to time
or when there is a change of ownership. A signed statement of intent by the owner
to the local police firearms licensing department should be sufficient to effect the
necessary change of status when required. A variation fee would become payable
where an ‘antique’ is brought onto certificate to allow it to be fired, unless a ‘one for
one’ variation is sought. In the latter case, it should be borne in mind that mostly
only mass-produced muzzle-loading arms had standardised bore sizes. Therefore,
a variation for a craft-made muzzle-loader may require finding a suitable example
before the calibre can be ascertained. As this may take some time, some latitude
may be given over the time taken for such ‘one for one’ variations.


Ammunition

8.12 Ammunition does not benefit from the exemption for antique firearms, and the
possession of live ammunition suitable for use with an otherwise antique firearm
may indicate that the firearm is not possessed as a curiosity or ornament.