Cooper, who lives in Gloucestershire, admitted he had been dreaming of such a find ever since he took up the hobby.
“As soon as I saw the abstract Celtic horse design on the coin, I just dropped to my knees,” he said.
“When you’re feeling it in your hands, it is fairly indescribable to be honest, because these are only things you ever see behind glass at a museum.”
Bell, an assistant director who met Cooper while working on This Country, said: “There were some swear words, but it was more like he told me off, in a nice way – like an angry school teacher.”
That was because he was “quite nonchalant and casual” about the discovery, said Cooper, telling Bell: “It felt lost on you, in a way”.