[2][3] His controversial exchange with the Portuguese historian Jerónimo Osório attracted international attention partly on account of the scholarly reputations of the protagonists.
A reformer in religion, with Matthew Parker, then master of Benet College, he acted as an executor of his friend Martin Bucer, and both delivered orations at his funeral in March 1551.
Haddon and John Cheke were chiefly responsible for the reform of the ecclesiastical laws, prepared under Thomas Cranmer's superintendence, and with the advice of Peter Martyr, in accordance with an Act of Parliament of 1549.
A bill introduced into the parliament of 1552 for the renewal of the commission was not carried, and Edward's death put an end to the scheme, but Haddon and Cheke's Reformatio Legum Ecclesiasticarum did eventually appear in print in 1571.
[7] At Michaelmas 1552 the king and council removed Owen Oglethorpe, President of Magdalen College, Oxford, who was opposed to further religious changes, and Haddon was appointed to succeed him.
Some libellous verses against the president, affixed to various parts of the college, were attributed to Julins Palmer, who was expelled on the ground of "popish pranks".
The following day letters were received from the Queen commanding that all injunctions contrary to the founder's statutes issued since the death of Henry VIII should be abolished; Haddon having retired, Oglethorpe was re-elected president on 31 October.
[8] In 1557 he translated into Latin a supplicatory letter to Pope Paul IV from the parliament of England, to dissuade him from revoking Cardinal Pole's authority as legate.
[7] In 1560 a Latin prayer-book, prepared under the superintendence of Haddon, who took a former translation by Alexander Alesius as a model, was authorised by the queen's Letters Patent for the use of the colleges in both universities and those of Eton and Winchester.
In June 1562 he and Parker, at the request of the senate, induced William Cecil to abandon his intention of resigning the chancellorship of the University of Cambridge.
He was at Bruges in 1565 and 1566 with Anthony Browne, 1st Viscount Montagu and Nicholas Wotton, in negotiations for restoring the commercial relations between England and the Netherlands.
[7] In 1563 Jerónimo Osório, a Portuguese priest known as a historian, published in French and Latin an epistle to Queen Elizabeth, exhorting her to return to the communion of the Catholic Church.