What did King Henry VIII become the Supreme Head of?

The Church of England

In May 1532 the Church of England agreed to surrender its legislative independence and canon law to the authority of the monarchy. In 1533 the Statute in Restraint of Appeals removed the right of the English clergy and laity to appeal to Rome (The Pope), on matters of matrimony, tithes and oblations. It also gave authority over such matters to the Archbishops of Canterbury and York. This finally allowed Thomas Cranmer, the new Archbishop of Canterbury, to issue Henry's annulment; and upon procuring it, Henry married Anne Boleyn. Pope Clement VII excommunicated Henry VIII in 1533.

In 1534 the Act of Submission of the Clergy removed the right of all appeals to Rome, effectively ending the Pope's influence. The first Act of Supremacy confirmed Henry by statute as the Supreme Head of the Church of England in 1536. (Due to clergy objections the contentious term "Supreme Head" for the monarch later became "Supreme Governor of the Church of England" - which is the title held by the reigning monarch to the present.)

Such constitutional changes made it not only possible for Henry to have his marriage annulled but also gave him access to the considerable wealth that the Church had amassed.