http://content.met.police.uk/Site/firearmslicensingfaqs
Unexpected possession of a firearm – what should I do?
You are advised to contact either your local Police or Firearms Licensing Authority as soon as possible. Avoid placing yourself in danger or in jeopardy.
If you find unlicensed weapons, for instance discovering them in your loft having just moved into a new address, do not handle them - they could be loaded and in a dangerous condition. Immediately call your local police who will remove them safely and dispose of them for you.
If you are left with firearms that you know had been legally held (this could be due to the death of a Certificate holder, who was a close relative) you have several options.
Temporary Permit - You could request a permit to allow lawful access to the weapons in order to give you time to decide the best means of disposal. The Police are empowered in such circumstances to issue a Temporary Permit, which would normally be restricted to possession only (not use) and be valid for a limited period of time, usually a month, but in certain circumstances can be longer. To request this, you need to contact the Firearms Licensing authority.
Registered Firearms Dealer - You could arrange for them to be placed with a Registered Firearms Dealer for storage and/or sale.
Certificate Holder - You may know of other Certificate Holders to whom they may be transferred (this would depend on them having sufficient storage and, in the case of guns that had been held on a Firearms Certificate, the authority to acquire that particular type of weapon).
Deactivation - Another alternative is to have the weapons ‘deactivated’, which would render them safe and remove them from all legislation relating to firearms. They would still retain the original appearance but would be incapable of discharging a missile.