It falls under the Treasure Act of 1996 whereby you have to reveal your 'treasure', which I suppose relates to all metal objects. Valuation is set by a committee.
How do you value bronze axes? could be the current market of 'acquiring' heritage from the past, big question should these bronze age axes remain in this country and be brought by a museum, or should they go on the open market.
Who owns national treasures, no one, they are a 'common' inheritance to be passed on....