The Romans had a fairly foolproof way of converting new lands to Christianity. Kill everyone who professes a different belief. So it was fairly hard to carry on being a druid, and why it pretty much died entirely but for a few remnants eaten by Christianity (the fertility rituals of Easter, for instance, owe a lot to the Pagan religions of Europe) and traditions passed down through word of mouth, devoid of their religious significance.
Druidism has, of course, recently seen a resurgence. I think it's a bit silly, to be honest. We know so little about the ancient religion that those who practice it seem to me to be going through rituals they think their forebears might have done, largely in ignorance of what their significance was or whether they're doing them in any way similarly to the originals.
Interesting note - the Romans Christianised the Isle of Wight last. I don't know why they left it out as they encroached Northward. Maybe they didn't notice it. Anyway, they converted that island by slaughtering everyone living on it. Charming people.
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