It's quite true that there is no archaeological evidence of Druids in Britain - in contrast with the clear evidence from Gaul. But can Tacitus' account in Annals 14:30 be dismissed so easily? Caesar's reference to druidism having its origin in Britain is fairly dubious, but considering Tacitus' father-in-law was Julius Agricola, who served under Suetonius Paulinus at the time of the Roman assault on Mona, perhaps Tacitus had a reliable source for his account.

He explicitly says that druids were present at the battle on Mona. Do we have good reasons to assume he was mistaken?