Meanwhile, as others have said, beware of ANY explanation that cheerfully tells you what the Druids or pre-Roman Britains did. WE DON'T KNOW.

We have some slanted reportage from the Romans, who had good political reason to demonize their opponents as practicing ritual sacrifice et al. And we have stories written down during the Christian era, and odd ceremonies performed during the Christian era, that we THINK contain hints of the previous culture. And we have archaeological artifacts that we THINK we can guess the purpose of.

But we don't have direct proof of almost any of it. So if someone says "The Druids did so-and-so", or "The Goddess Rhiannon was such-and-such," ask 'em for documented proof that dates back to the relevant era, and don't accept references to other modern mythologists as meaningful.