strip the gun down for spares to sell(magazine,bolt,stock,screws,cartridges,trigger parts) and destroy the receiver and barrel then dispose of whats left of the receiver/barrel.
don't take it to the cops in trying to do the right thing,it can end up with you being questioned and an attempt to prosecute,there are home office guidelines by which the police should follow but thats all they are...just guidelines not rules.
sadly air cartidge guns are now non transferable past a certain date.