I think it's worth remembering that at the time Britain was already a constitutional monarchy. King George may have been head of state and could officially veto anything brought to him by parliament but the real power lay with the house of commons and the house of Lords. The party in control of the house at the time were the Tories who would be thought of in modern terms as a conservative group concerned with maintaining status quo and landowners rights. Their opposition were the Whigs who mostly represented the burgeoning middle class and more progressive attitudes. The Conservative response to Declaration of independence was to see the colonists as ungrateful and that this insubordination should be put down by rule of law. The Whigs however were split by the idea of military action. Many people were appalled at the idea of military action against people who were thought of, and identified themselves as, British citizens. As the war went on and became increasingly bitter support for the war dwindled. Eventually the Tories lost control of Parliament and the Whigs negotiated the treaty of Paris.

There were at least two parliamentarians who showed support for the American Revolution; John Wilkes and Isaac Barre. Both spoke up frequently in Parliament in favor of American causes, and Barre coined the term "Sons of Liberty" in one of his speeches.