Crime to find a coin

Apparently it is a crime in the UK to find a coin:

A woman who found a 700-year-old coin in her garden as a child has become the first person to be convicted of failing to hand in suspected treasure.

Kate Harding, 23, was prosecuted under the Treasure Act after she ignored orders to report the rare find to a coroner.

The article is a little misleading because the prosecution was based on the object being not really a coin, but some sort of coin-like metal token of unknown purpose. But still, why should it be a crime to find a piece of metal in your yard? And why tell the coroner? I thought that a coroner was an official who keeps track of dead bodies.