Just hand it in to the coppers if you don't want it anymore. They ain't worth shit anyway, unless it's some rare thing.
When I emigrated, I had an old BSA Meteor. I thought about selling it or just dumping it at the tip, but I didn't want it on my conscience that some 17yr old scrote would have someone's eye out with it. So I took it to the Police, explained my story, and they seemed genuinely grateful. No charge, no hassle.
As my veneered colleague Beerhunter so succinctly points out, the precedents are there and well-documented of guns having been handed in to various police forces turning up in, ahem, unexpected places. I recall a story, perhaps now an urban myth, of a significant number of handguns, having been handed in at the last Great Betrayal, re-appearing in criminal hands.
However well-intentioned police forces are, just like the rest of the population, they are made up of folks who represent a cross-section of the community.
It only needs one small turd floating in your soup to put you off the rest of your dinner.