Tiny nitpick, but you need to find two coins to qualify:

https://en.wikipedia.org/wiki/Treasure_Act_1996

I think that's the right call. A common person can find an old coin and not really think about the historical impact it has. Merely keep it on his or her shelf or bring it to a shop for some extra cash.

But once you're in the realm of finding multiple coins, the onus kinda changes. There may be more, greed sets in and wanting to conceal the find is bad. Also, England doesn't "take their treasure" it buys it off of them if the museum wants to buy it and for a fair price.