You could apply for a Section 7 temporary certificate, that would allow you to dispose of it via an RFD. Do a search on BASC for unlicensed firearms and bereavement. HO guidelines say 'no obstacle shall be placed in the way of someone who wants to surrender firearms or ammunition to police', even so its an offence to possess unlicenced, a S7 would allow you to sell it.
"I'm all in favour of keeping dangerous weapons out of the hands of fools. Lets start with typewriters." - Frank Lloyd Wright (1867-1959)