Besides the obvious fact that the druids didnt have any real magic, there are some places were the series takes some rather huge liberties with the historical facts. When asked if his story would enrage pedants, Britannia creator Jez Butterworth actually responded that "I actually hope it does."
- Pre-Roman Britannia being a Feminist Fantasy World of Action Girls is a greatly exaggerated view of what we know for sure about the roles women occupied on it. While it is true that Celtic women had a great share of independency, could occupy political and religious positions, and stepped into the battlefield with a frequency unheard of at the time, those customs were actually heavily dependant on the tribe in question (and on circumstances) and certainly not as deep and well established as the series shows.
- The Britannian Celts from the series use runes. Nevermind that the runes were a Germanic writing system and did not arrive to Britannia until the Anglo-Saxon migrations, 400 years after the events of the show. If the runes were around in 43 AD (the earliest known inscriptions are from the 2nd century), they would have been limited to the area around Denmark or to Germanic mercenaries.
- The writers seem to have gone for the As Long as It Sounds Foreign route, because aside from maybe Antedia, none of the Britons have period-appropriate names. Their names are a mixture of anachronistic Gaelic names (like Ossian and Phelan - technically Celtic, but only very distantly related to the Brythonic names that the historical southern British Celts would have sported), foreign words (like Amena and Divis, which come from Latin of all things), and just plain random-sounding names (like Quane or Willa). The worst example might be Lindon, who sports a centuries-posterior Old English name despite the fact he is not a even a Briton.
- Characters played by black actors are identified as "Numidian", an olive-skinned Berber people from North Africa. It is possible that the screenwriters might have confused the Numidians with the Nubians, who did have dark skin, but the first legions in proximity to Nubia wouldn't be raised until the reign of Diocletian, 250 years after the series takes place.
- Antonius claims that the god of the Numidians is Bomazi. In reality, Bomazi has nothing to do with Numidia: it is the god of the Bushongo people, which were located in Central Africa, a place the Roman empire never got remotely near of. For a Numidian deity, the goddess Tanit would have been more adequate.
- In the series, Roman citizens are apparently expected to abandon their native deities and worship the Roman gods, as a plot point is made out of Antonius having always been secretly worshipping Bomazi despite being now a Roman soldier. In real life, Romans could hardly care less about what gods their people prayed to, as it was their belief that all the people in the world worshipped the same gods under different names (interpretatio romana). In fact, foreign theonyms like Mithras were very popular among Roman soldiers at the time the series is set.
- Lokka is an alternative rendering of the name of the Norse god Loki, not a Celtic "Earth Demon", which is also a mythological concept invented by the show.
- The Romans are seen drinking wine out of modern glass goblets, instead of ones made out in the style of the period.
- Basically every costume in the series is comically wrong. Eagle-eyed viewers may notice that some of the officers' helmets have been purchased from cheap £1 party stores.
- The Romans have Czech hedgehog style traps surrounding their camp, 1895 whole years before they were invented. Amusingly enough, those traps were designed to stopping tanks and vehicles, not infantry, so ancient Romans would have probably not found any real usage for them.
- All the Britannian characters, either main or secondary, are fictitious, and some of them are stand-ins for historical figures. Queen Antedia replaces the historical king Cogidubnus, while King Pelennor and Kerra do the same for Togodumnus and Caratacus, the Catuvellani chieftains who ruled over the Cantii at the time (like Pelennor, Togodumnus died during the Roman invasion, and like Kerra, Caratacus inherited his job in the resistence).
- It can be inferred that the Roman officer Vespasian is meant to be the historical would-be emperor Vespasian, who actually served in Britannia as an officer under Aulus Plautius. However, his Britannia version dies there and never becomes anything.
- The "arrangement" between Lindon and Amena is said to be the druids' idea in order to strengthen the bonds between the Cantii and the "Bretons" in Gaul. There are two problems with this: at that point in time the peninsula of Armorica had been part of the Roman province of Gallia Lugdunensis for decades, and it wouldn't be called Brittany for another four centuries, when Britons from the island of Great Britain began to emigrate there.
- Plautius reminds Lucius of an incident in which they were both involved while serving in Egypt, and mentions Cairo of all things. While the area was already home to various cities and settlements, Cairo wouldnd get that name until the 10th century AD.
- Plautius claims to not even know the names of the local tribes in the area where he landed. In reality, the pretext for the invasion was to re-instate Verica, an exiled king of the Atrebates tribe, so he should have been well aware of the political situation on the island.