Arriving in England, Barclay was offered considerable preferment by James VI on condition of becoming a member of the Church of England. This offer he refused, and he returned to France in 1604, when he was appointed professor of civil law in the university of Angers. He died at Angers in 1608.
Works
William Barclay’s principal work was De Regno et Regali Potestate (1600), a strenuous defence of the rights of kings, in which he refutes the doctrines of those he terms monarchomachs: George Buchanan, "Junius Brutus" (Hubert Languet or Philippe de Mornay) and Jean Boucher, a leading member of the French Catholic League; he also wrote De potestate papae: an & quatenus in reges & principes seculares jus & imperium habeat (published in 1609, after his death), in opposition to the usurpation of temporal powers by the pope, which called forth the celebrated reply of Cardinal Bellarmine; also commentaries on some of the titles of the Pandects.
"William Barclay, Professor of Law at Pont-a-Mousson and Angers" by Andrew F Stewart in Stair Society Miscellany V, ed H L MacQueen, Edinburgh 2006 ISBN1-872517-18-8
Andrew Pyle (editor), Dictionary of Seventeenth Century British Philosophers (2000), article pp. 59–62.