This is probably subject of a lot of historical debate without anyone coming to any clear conclusion due to lack of evidence ie the celts unlike the Romans did not keep any written records.
The only evidence I have seen on the various history channels regarding drugged up warriors ( often the best) are those that they sacrificed to the gods and then the body was dumped in a bog which has preserved the body to enable us today to confirm the fate of this person.
What Britain did have in its favour was a higher perecentage of druids who were responsible of such acts so it could be some warriors were fanatics but clear evidence is lacking. The chance of whole units being fanatics is probably even slimmer but who knows? If I was doing a British warband army I don't think I would have fanatics but I would certainly have a unit with a druid in it. Jusy my thoughts on the subject.