There will still be shotguns probably all over the country that aren't registered that hark back to the days when you didn't need a certificate to have a gun. Old farmhouses and the like from ancient relatives who missed the memo about registering them.
If one comes into your hands, so long as you inform the police, you'll have nothing to worry about. And the police, despite more and more legislation being enacted have yet to find a way of prosecuting the dead. They tend not to appear in court. You can hand it in or convey your wish to keep it and be advised by them of the best way to do that, there's a few ways of doing that. Just be open and honest and what you wish for will come true (and you can't say that about many things in life).