“Peace, prosperity and friendship with all nations”: what a curious motto for the new 50p coin that the Treasury has announced will commemorate Britain’s exit from the European Union. It has an air of playground desperation about it – the neediness of a kid who, having wilfully pissed off all 27 of his friends, is slinking nervously back into the schoolyard looking for someone – anyone – to play with.

It will not be the first time that Britain’s relationship with Europe has been marked on its coinage. Commemorative 50p pieces were struck in 1973 when Britain joined the European Economic Community, and in 1998 to mark the UK’s presidency of the EU. But you could argue that the neatest parallel to the Brexit coin goes back almost two millennia, to the third century AD and the first, dramatic Brexit – under Britain’s breakaway Roman emperor, Carausius.

A Roman officer from modern Netherlands, Carausius was given command of a fleet whose job was to rid the Channel of Saxon raiders. But, accused of colluding with the pirates, he was sentenced to death. His response, in about AD 286 or 287, was to seize control of the Roman province of Britannia, along with a chunk of France around Boulogne.

Precisely what was really going on here is hard to unpick; there is hardly anything to go on in the historical record (a fact that probably helped rather than hindered the writer Rosemary Sutcliff in fleshing out a wonderfully compelling account in her children’s book The Silver Branch). But there are strong clues in the distinctive, highly suggestive coins Carausius struck. One declared him “the restorer of Britain”, another “spirit of Britain”. Another was inscribed with words that translate as “Come, awaited one” – an adaptation of a phrase from Virgil. Another had an image of Romulus and Remus suckled by the she-wolf – the great, resonant image of Rome’s mythical origins – with the inscription, “the restorer of the Romans”, along with the rather enigmatic initials RSR. Carausius was claiming to hark back to a better, stronger age.

However, the most intriguing coin he struck was a splendid gold medallion, the sole example of which is in the British Museum, and which I was (thrillingly) allowed to examine and hold when researching my book about Roman Britain, Under Another Sky. The coin, brought into the museum by a little boy in the 1930s, shows Carausius himself dressed in a consular toga (pure fake news, incidentally, since he was never elected consul). The reverse shows Victory in a chariot and the words “victory of Carausius Augustus”. Also on the coin are the letters INPCDA.

For years no one had a clue what the letters RSR or INPCDA alluded to. Then, in 1997, it suddenly occurred to the writer and broadcaster Guy de la Bédoyère – while running a bath for his children – that they could perhaps be something to do with Virgil. On the offchance, he flicked through the Oxford Dictionary of Quotations. And there the letters were, right in front of him. RSR stands for redeunt saturnia regna, or “the Golden Age returns”; INPCDA for iam nova progenies caelo demittitur alto – “now a new progeny is sent from heaven”. Together, they are the most famous line and a half of Virgil’s Fourth Eclogue – an ecstatic poem foretelling the start of a glorious new era.

Carausius’s “Brexit” coins, then, can plausibly be seen as a claim that he will preside over the rebirth of Britain – coupled with a bold, not to say shameless, assertion of legitimacy. (By pure chance, until the round British pound coin was replaced with a dodecagonal version last year, it also had a quotation from Virgil inscribed on its rim – decus et tutamen, meaning ornament and protection, which comes from the Aeneid.)

It strikes me that the leading Brexiteer Boris Johnson, for one, might like nothing better than to have himself minted on to a coin wearing a toga and surrounded by quotations from Virgil announcing him the harbinger of a new golden age. But the rest of the story might put leavers off a bit. In around 293 Carausius was assassinated by a shadowy figure called Allectus, who assumed control of Britannia. And then, in 296, the general Constantinus Chlorus arrived with troops and brought the province back into the empire (he was the father of Constantine the Great, proclaimed emperor in York a decade later). Britain’s first Brexit, as far as we can tell, was a short-lived and unhappy affair – whatever the coins might have claimed.

• Charlotte Higgins is the Guardian’s chief culture writer