Brit govt probably took half.
I believe that - in the case of buried treasure - the entire find is confiscated by the government, and the finder is consoled with a "Finder's Fee."
In the case of valuable found on or in an actual residential building - I don't know.
Might explain why "buried treasure" is so seldom found - but instead troves are discovered hidden in the walls or concealed in the floorboards.
Regards,
To preserve archaeological finds and information, anyone in GB who finds a “treasure” has 14 days to report the find. It then gets offered for sale to several museums. The finder gets the proceeds. If no museum is interested, it is returned to the finder.
So, the finder can actually get pretty close to market value for their finds. The US government taxes it as ordinary income in the year you found it. So I’ve read.