Answer: Parliament
Here are some key moments in the history of the growing power of Parliament in English history:
The Magna Carta (1215) asserted noblemen's rights in relationship to the king. It set the principle of rights which would later be expanded.
The English Civil War (1642-1651) was a battle between Parliamentary forces and the armies of the king, because of a perceived overstepping of power by King Charles I. Charles was executed and Parliamentary forces (led by Oliver Cromwell) came to power.
The Bill of Rights (1689) was an agreement made with King William III and Queen Mary II as they came over from the Netherlands to take up the royal throne of England after the so-called "Glorious Revolution" of 1688. It limited the power of the monarch and gave greater authority to Parliament, essentially setting up England as a constitutional monarchy (rather than an absolutist rule by a monarch).