The good news is that as the gun was deactivated/came into your ownership so long ago, you probably paid very little for it compared to today's prices. I would estimate that the retail on that from a dealer would be £600, so a trade price for that from a dealer would be 300-350 (they have to make money to), so you are quids in anyway. The advantage of selling to a dealer (section 5 one), is they can take it off you in the current state of deactivation, so you don't need to worry about getting it redone to the latest eunuch-like level to sell it.

Happy days!