I am in possession of a 1906 King's Prize Medallion won by my Grand father. The Medal says that it was issued to the "National Artillery Association. Shoeburyness" - Can anyone tell me what this medal was for - or what the value may be

The National Arrtillery Association was formed about the time the National Rifle Association was formed and for the same reasons - to foster and improve Artillery Shooting amongst the Volunteer Forces Artillery.  It exists to this day but from what I can see is now almost exclusively for the Regular Artillery, as TA artillery, as a separate organisation, scarcely exists.