Maybe not. Doesn't the UK gubmint usually take/seize a large portion of finds like this? If that happens it will keep most of them off the market.....
Depends upon your definition(s). It works out to the equivalent of Eminent Domain here in the USofA, the Government is taking this but has to offer the "Market Value" of that which they are taking. Obviously the devil is in the determination of the value. Other considerations that come into play are the property owner's rights, the right to keep something and it's value as a historical artifact.
The feds in the UK treat treasure as the USA does eminent domain. They take the treasure but pay the finder a fee, whether it is a fair fee or not I don't know.