England became officially Protestant under Edward VI, but that really was just an extension of the changes that had begun under his father, Henry VIII.
Henry had famously begun the split with the Church over the issue of his divorce. Parliament then passed the Act of Supremacy, making Henry the sole leader of the Church in England. He was given power to, for example, set doctrine and appoint bishops. ONce this was done, England was essentially Protestant.
After Henry died, the split was made complete. Leaders like Archbishop Cranmer, who wanted to go farther than Henry would allow, were able to have their way. They did things like abolishing clerical celibacy and adopting a new liturgy for the church in the Book of Common Prayer.
These changes, of course, were not permanent. Edward died very young and was succeeded by his half-sister Mary, who was a devout Catholic and restored Catholicism for as long as she reigned.