I’m sure the Brit govt will pull out some act that denies these guys even a finder’s fee. They will confiscate it all. Then the EU govt will claim it’s theirs.
Probably The Treasure Act of 1996 covers their find. This specific designation of what "treasure" is: "Objects substantially made from gold or silver but are less than 300 years old, that have been deliberately hidden with the intention of recovery and whose owners or heirs are unknown."