Sky’s star-studded drama series, set among the warring tribes that inhabited Britain at the time of the Roman conquest, is written by Jez Butterworth, author of the hugely popular stage play Jerusalem. Fans of his riotously demotic and inventive dialogue and embodiments of rural, almost folkloric and mystical British spirit will find Britannia’s druggy Druids, solstice-loving Celts and mystical outsider figures familiar.