Hi,
I've no idea when the deactivation laws came into force or even if a certificate is needed, but without one from the proof house it would be very difficult to claim that it's been done to a UK standard.
Deactivated weapons are any firearms which have been converted, in such a manner that they can no longer discharge any shot, bullet or other missile. More importantly, deactivation is intended to be permanent and such firearms should be incapable of being reactivated without specialist tools or skills.
Deactivation work carried out in the UK since 1st July 1989 will generally have been endorsed by one of the Proof Houses, the weapon proof-marked and a certificate of deactivation issued. To these ends, any weapon, even a prohibited weapon such as a machine gun, can be deactivated. The outcome is that the weapon is no longer a firearm within the meaning of the Firearms Acts, and consequently may be possessed without a firearm or shotgun certificate and may be displayed in the owner's home, rather than be locked in a gun cabinet.
Deactivation of a firearm is not something to be undertaken by the layman. There are stringent requirements before a weapon can be proofed as deactivated and such work is best left to a gunsmith. A Registered Firearms Dealer is the best person to speak to if you require a weapon to be deactivated. He can make all the necessary arrangements for you, including deactivation of the weapon and getting it proofed. You should not "deactivate" a weapon yourself; for instance by filling in the barrel or filing off the firing pin. In such circumstances the weapon will still remain a "firearm" and require to be held on a certificate. Deactivation needs to be carried out by a qualified gunsmith in order to meet the standard necessary for proofing.
Although the above references to proofing and certification do not preclude the possibility that a firearm which has been deactivated in some other way may also have ceased to be a firearm within the meaning of the 1968 Act (as amended), it is important that care is taken when acquiring any firearm which is described as deactivated. You should ensure that you are shown the Proof House mark and deactivation certificate issued in respect of any gun deactivated in the UK since 1st July 1989.
Further advice may be sought from the Firearms Licensing Office, Registered Firearms Dealers or the Proof Houses.