Seems to be the case that the disposal does need to come first -
“One for one” variations
10.36 A variation is always necessary if a certificate holder wishes to change one of the firearms, even if they wish to purchase one identical to the one they are selling (Wilson v Coombe, Queen’s Bench Divisional Court, July 1988). “One for one” variation refers to firearms that are authorised to be acquired following the disposal of a firearm or a request to change an existing authority to acquire. There is no set time in which the certificate holder must apply for a replacement authority once their firearm is disposed of. The keeping of open authorities indefinitely should be discouraged, subject to a collector seeking particular firearms. Applications for “one for one” variations should be made by the certificate holder submitting their firearm certificate together with a completed Form 201V to the police firearms licensing department. Such variations are processed free of charge. In most cases, it will not be necessary to re-examine the applicant’s circumstances. Further enquiries will be necessary, however, if for example the application is for a change of use or for a full-bore firearm when the holder’s shooting club only has facilities for small-bore shooting.