Below is a link to the Quebec Act which basically gave that province to the Roman Catholic church. It is my understanding that there was quite a debate in the House of Lords concerning this legislation. England still had a mistrust of anything Catholic. But,they were concerned about the possibility of an American invasion and needed the support of the French Canadians in the event of a war. By giving ecclesiastical authority to the Catholic church they gained the considerable influence the church had upon the french Canadians and natives. The Catholic church would feel as threatened as the English did with a possible American invasion.
http://www.solon.org/Constitutions/C...n/qa_1774.html
For the rest of us of course the King was also head of both the church of England(Anglican) and the church of Scotland(Presbyterian). Also, it was an act of Parliament which formed the United Church; the largest Protestant church in Canada. The United church formed by the joining of three separate denominations but, I don't remember the three off the top of my head. In Quebec one of the perks of being the official religion of the Province was that they ran the public school system with funding from the Province. I believe there are a couple of other eastern provinces the have or had the same arrangement with a denomination.