Several men who went against the Anglican Church have feast days, which is what Thomas More was given. John Wesley, for instance, one of the founders of Methodism and hardly a unquestioning supporter of Anglican religious practice, has one. Caught in a very difficult political situation, More chose principle over political expediency. His intellectual achievements also would place him on a short list of great Englishmen. I suppose the short answer would be that the standards for veneration in the Anglican Church are not as rigid as they are for canonization in the Catholic Church, so More's accomplishments more than trump his opposition to the founding of the Church of England in the first place.