Home Office Guidance to the Police says as follows:-
Possession only of any type of firearm or ammunition without a certificate Permits authorising possession 6.17 Section 7 of the 1968 Act enables a chief officer of police to issue to a person a permit authorising them to possess a firearm or ammunition to which section 1 of the 1968 Act applies, or a shotgun, in any special case where it may not be necessary or desirable to issue a certificate. For example, a permit should in normal circumstances be issued to authorise the temporary possession by a relative or the executor of a deceased person, or the receiver of a bankrupt’s estate, of firearms or ammunition forming part of the property of the deceased person or bankrupt. A permit may not be issued to authorise the purchase or acquisition of firearms or ammunition (but see section 5(1) of the 1988 Act and chapter 15). If it does not specify possession only, it will be assumed by default that the permit allows use. A permit should be issued if a certificate holder’s firearm certificate or shotgun certificate has expired and they have made reasonable efforts to renew in good time.
So you could give your Executors a sealed envelope to be opened only in the event of your death giving the location of the key. If your wife is an executor then perhaps ask your solicitor to keep such a letter. The Executors can be given a permit to deal with a sale at a sensible price - NOT your FAO!