This is what Wikipedia has to say about him:
"According to Breton and French Catholic sources Saint Arthur (Arzur) of Glastonbury (d. 1539) was a member of the English Roman Catholic faithful in the 16th century. He was martyred during the period of King Henry VIII's suppression of the Catholic Church due to his refusal to accept the king's claim to spiritual leadership of the Church in England....English Catholic sources lack information on St. Arthur of Glastonbury's martyrdom under Henry VIII.
St. Arthur's feastday is celebrated regionally on 15 November in Brittany."
Fr. Vincent Serpa, O.P.