There’s a committee[0] and I guess Roman coins are one of the easier artifacts to value as there actually are public markets for them[1].

If you, as a finder, disagree with the valuation, you can “send your own valuation for the committee to consider”, which sounds like an interesting process to navigate. I imagine you’d have to find a credible alternative buyer who’d be willing to pay a higher amount.

[0] https://www.gov.uk/government/organisations/treasure-valuati...

[1] https://www.vcoins.com/en/coins/ancient/roman-coins-2.aspx?c...