If you have one that is still 'live' you commit an offence unless it is so old as to be an obsolete calibre - everything from the boer war onwards is still considered shootable.
Contact a proper gunsmith and explain the circumstances, he may accept it if he has a section 1 certificate but he will probably tell you to lodge it with the Police. Do not sign the destruction order and tell them that a gunsmith will be collecting it for de-activation and that you wish to keep it.
The NRA at Bisley have a gunsmith who has a lot to do with classic arms and he may be the one to assist you - google 'NRA Bisley' and you should be able to get a phone number.
I would advise you not to start wandering around the streets with it because, as I said, they are prohibited.
The gunsmith will do the releveant work and submit it to a proof house for the certificate to be issued.
It won't be cheap.
Do you know what it is?