Exactly who and what the druids were is the subject of much debate (and even more whacko theorising), but I think most scholars would agree that they were more likely a sect or a religious/intellectual class within Gallic society rather than a "culture".

And, as I recently pointed out in a post here which has (mysteriously) disappeared for some reason, there's no evidence that any druids survived more than a generation or two after Caesar's conquest of Gaul, so by Carausius' time the druids would have been a 250 year old memory, if that.

But we haven't actually discussed anything about Merlin yet ...