I’d say they’d still be valuable, and here’s why.
Before the Dark Ages (Middle Ages, the Medieval Times, etc.) was the Roman Empire. They conquered the known world through force. But what of trading? It would be hard, maybe impossible to force everyone in the Empire to use one type of coin. So it’d be easy to think they’d accept a coin regardless of who’s face was on it but rather dependent on the metal itself.
I expect there to be backlash and comments on my theory, but I believe that no matter the currency of the time and it’s worth, I think it’d be accepted if it was of the correct metal regardless if it came from London or from Prague.