Under the Treasure Act, I believe the British authorities are required to pay far market value for any items that are declared by them to be “antiquities.”
I remember reading here on Free Republic about a cache of Roman coins found in England. The British authorities paid the land owner and the metal detector enthusiast the equivalent of millions of dollars.
Perhaps I was hasty.
I am shocked if Britain gives finders rights.
Apparently they do, if you give them HALF.