LONDON — Conservative leaders within the Anglican Communion have signed a letter urging the archbishop of Canterbury to rethink his personal views on homosexuality, and to crack down on the U.S. Episcopal Church and the Anglican Church of Canada. The letter to Archbishop Rowan Williams, dated Wednesday, bore the signatures of a number of conservative Anglicans, including Archbishop Peter J. Akinola of Nigeria, who have accused the U.S. and Canadian churches of violating Biblical teaching by affirming gay relationships.