It's not a 'government guidelines' it's law! For those interested https://en.wikipedia.org/wiki/Treasure_Act_1996
> When treasure is found, the finder does not own it, and it is illegal to try to sell it, according to government guidelines.
There could well be guidelines that say that, about the law ('it is illegal', as it says). HMRC has plenty of guidelines on what tax you owe and how to pay it etc. for example, but it also comes down to law at the end of it.
[0]: https://www.gov.uk/government/publications/treasure-act-1996...