Rather inaccurate, but certain elements of it are disputed (like the historicity of King Arthur) but other elements such as the existence of a group of Trojan expatriates fleeing the Trojan War from the Iliad led by a Brutus of Troy colonizing Britain are regarded as mythological as there are no classical accounts of this ever happening (and the Iliad itself is a legendary work). As well as the author tracing Trojan family lines back to Old Testament figures (despite them historically being associated with pagan Greek deities) and the Christianization of Mediterranean pagan traditions in general is concerning. There are also several false etymological roots for Celtic and Gallic words and names. And it misrepresents Celtic and Gallic culture, tradition and history, and it uses the Roman origin of Arthur which is an annoying historical myth for me.

But Roman and Hellenistic era history is my strong suit so perhaps someone who has a focus in pre-Roman British history can provide a better answer.