Harrison seems to have misunderstood his source: Demetrius does indeed describe the islands of Britain as here, but as a participant in a discussion, not as a source, and he is not himself said by Plutarch to have been British - Opera quae extant omnia Plutarchi Chaeronei, Ethica sive Moralia complectenti (Frankfurt-am-Main, 1580), 459 (in the treatise Quare expiraverunt oracula).