It would probably crash the price of the other current set by £50,000 as soon as someone "took" an additional set out of the ground legitimately. All collectible are valued on demand and supply, and to get a heist like this you probably can only get rid of the "payload" for £10,000 if you are lucky, and that means the work must be performed cheap. Digging up a foundation would probably be more expensive than that even if you are doing it quick and dirty. This set of coin is safe simply due to it being not a profitable business to steal it today.