Could he have avoided being in possession? Yes, easily.
He's not some blushing virgin who's never had any dealings with the police. The guy is a convicted violent criminal who is not unknown to carry a weapon.
Depite this, when he 'found' a sawn-off shotgun did he decide to not touch it with a bargepole and to just call 999? No.
Instead, he took it home. He then kept it for a few days before eventually deciding to give it to a cop he knows. He didn't invite the cop to his house, he arranged to meet him elsewhere and took the sawn-off out in public again.
Is there some reason the guy likes walking around tooled up? Having already been done for it once, he doesn't seem to have gone out of his way to avoid it again.
If nothing else, the guy is a complete spanner. At best he got himself nicked through his naivety.