Let's run through this story again...
Young lad receives a metal detector for Christmas (the British version of xmas).

The next day, Boxing Day, his dad takes him out somewhere to try it out.
Dad chooses a nearby protected nature reserve, where the rules are so strict you can't even step off the designated path lest you squash a special blade of grass etc.
They start digging on signals on a site they don't have legal permission to detect on, let alone dig.
Said site is also a former bombing range, the closure to the public for years it was in use creating the habitat that led to the nature reserve being established.
Solid iron practice bomb found.
Must have been a slow news day.