Nonie,

we do have some archaeological evidence about Celts and druidical practice--including animal sacrifice--although most of it is from Gaul, not Britain. We also have archaeological evidence of human sacrifice among Celts, although not from the region where druids are attested. Druidic human sacrifice is attested in Classical accounts, but not much in the archaeological record. However, many archaeologists think that various bog bodies fit the bill as evidence of human sacrifice among Celts who would have practiced the religion overseen by druids. In general, this evidence adds up to support a position that it is likely, but not certain, that druids practiced human sacrifice.

What happens in the more fuzzy world of pop archaeology and pop history is that people conflate all the evidence from the Celtic world (which, of course, covers many centuries and a wide swath of Europe and Asia Minor) into one distorted picture of "the Celts." That is problematic. But it's not true that we know NOTHING about them, either.