There must have been shamans leading the tribes of Britain before the Roman invasion in the first century CE because they were all over the world, although not necessarily called shamans, which is a Siberian word.
According to their oldest stories, the Celts had their Dreamers, or awenyddions, and you can enjoy readng about them in book The Grail Mysteries.
But for those with eyes to see it, the stories contained in these myths are clearly built around much older templates that have common themes with ancient cosmological myths, some dating as far back 3,000 BCE. This doesn’t seem such a bizarre claim now that we know that our myths that were orally passed on by our ancestors who were Scythians, a nomadic tribe that swept down from the Caucusus Mountains after the last Ice Age, spreading to all parts of the Northern Hemisphere. Increasingly, historians are becoming convinced that those Scythians that went southwards towards the Mediterranean became the Celts, and those that went north became the Saxons and the Vikings. Continue reading