The usual confusion between guidelines and law seems to be creeping in here
From the HO guidelines at;
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/117636/firearms-security-handbook.pdf
You can find:
PART 2 – SECTIONS 1 AND 2 FIREARMS (FA1968)
2.5 Under most circumstances,
it is preferable that firearms should be secured within theoccupied part of the structure. Separate, detached buildings, or those attached but having onlyexternal access (eg outhouses, garages etc) should not be used
unless the levels of securitywarrant it. If used, these
could also be protected by an intruder alarm linked to the household.
Nothing in section 2.5 is mandatory, but we all know how FEO's can read guidelines as laws........
There are quite a few areas open to interpretation just in the quote above (bold, U/L and italics)
So, in answer to the OP, if your shotgun cabinet in the garage is up to spec, and has been previously used to store firearms/been approved then why not use it.
Otherwise, be prepared to explain to or negotiate with your FEO if you seek guidance.
Best of luck with the storage and subsequent sale
Geo