The UK has very strict and peculiar laws regarding finding cultural artifacts.
I believe it is more complex than that. Something lost is one thing, something hidden or buried is treasure trove. Which is which is determined, I believe, by the coroner. Treasure trove belongs to the Crown. But the Crown pays fair value for anything a museum might want. The rest goes to the finder and landowner. By paying fair market value, it encourages people to report finds which can be then be properly examined by archeologists, historians, etc.