My first thought seeing this is that the coin certainly is old and Roman but that does not mean it was lost anything like 200 BC. The state of preservation could suggest anything from pot find rather than straight from the soil to a loss some time in the last century out of a collection. This is where exact details of the find and appearance before cleaning would be nice. To be historically significant, it would be nice to link the coin to even a suggestion that it spent any time in the UK before Claudius. Does anyone recall a few decades back when a British/Roman (Allectus, I believe) coin was found along the St. Lawrence in North America? Some, as I recall, suggested it meant the Romans visited the New World in the 3rd century AD. Others suggested that fur traders used ballast from UK river dredging gravel for the trip to North America which they dumped when they loaded furs. Either explanation works. I have failed to find a write-up about this but that could mean my Google skills are lacking. If anyone has a link to the story, I would appreciate hearing.
The article quoted the British official who inspected and researched the coin: I don't know how many coin collectors there were in Britannia in 207 BC but I suspect the number was near zero. The evidence seems to suggest the coin was lost or buried (cached) not long after it was minted and brought to Britannia by the Romans. To me this is an amazing find.
Yeah, I found this part to be the most interesting: "Most Republican denarii found in Britain are very worn as they could be in circulation for up to 300 years." That's crazy!
Estimating circulation from amount of wear is an inaccurate business at best. But judging from the wear on that, it did not circulate much. This does not necessarily mean that it was deposited in 210 BC or anywhere near that. If I recall the record of debasements clearly, that coin could have been in circulation as late as the reign of Nero, several years AFTER the invasion of Britain. Its not at all infeasible for a denarius (or hoard thereof) to be lost, found, and subsequently placed back into circulation. With regards to North American finds of Roman coins: they mean absolutely nothing. For a coin find to prove Roman contact with the Americas it would have to be a full hoard of many coins, found in a stratified context in an archaeological site. Stray finds or anything pulled from context mean absolutely nothing in this case. Any evidence of ancient contact with North America before the Vikings is always sketchy at best and always easier to explain through other methods. Do I think that ancient ships and traders stumbled across this continent? Yes. I honestly do. Do I think that coins or anything else serves as evidence for this? No.
Interesting, if a coin is found with a detector in 300 years on Mars, will it be unmarsed? Just a thought.