(Not sure it this counts as Britarch-worthy, but) the most archaeological coin currently in circulation is probably the Gibraltar one pound coin with the Neanderthal skull on the reverse. For the information of our foreign colleagues it's not legal tender in the UK, but I found one of these in my pocket when putting together the cash to pay the bill in a restaurant the other day. Was I justified in including it as part of the tip?
My only excuse is that some months ago the same restaurant included a Channel Islands pound (Bailiwick of Jersey, I think it was) when they gave me my change.
However, I Googled it when I got home and discovered that a Gibraltar Neanderthal skull pound coin was on offer on e-bay for £2.90! So perhaps the restaurant could make a profit on it - if they even noticed.