Yes, there were American sympathizers in Britain during the American Revolution.

As with any such revolt, there will almost always be members of both the civilian populous and the government who supports the opposition. In this case, the Whig opposition to Lord North’s government had deep sympathies with the American colonists and their grievances, urged for an settlement with the Colonies, and when the war broke out, they were the first to question and criticize the military handling of the conflict.

Specific influential individuals of the populous held similar views, such as Edmund Burke, who voiced his sympathetic views to Parliament, and had a share of influence in the forcing out of Lord North.

Quote from “Washington and his Comrades in Arms, a Chronicle of the War of Independence”, “It is to the credit of Whig leaders in England, landowners and aristocrats as they were, that they supported with passion the American cause. In America, where the forces of the Revolution were in control, the Loyalist who dared to be bold for his opinions was likely to be tarred and feathered and to lose his property. There was an embittered intolerance. In England, however, it was an open question in society whether to be for or against the American cause. The Duke of Richmond, a great grandson of Charles II, said in the House of Lords that under no code should the fighting Americans be considered traitors. What they did was “perfectly justifiable in every possible political and moral sense.” All the world knows that Chatham and Burke and Fox urged the conciliation of America and hundreds took the same stand. Burke said of General Conway, a man of position, that when he secured a majority in the House of Commons against the Stamp Act his face shone as the face of an angel. Since the bishops almost to a man voted with the King, Conway attacked them as in this untrue to their high office. Sir George Savile, whose benevolence, supported by great wealth, made him widely respected and loved, said that the Americans were right in appealing to arms.”

A huge example of an American sympathizer in Britain was Charles James Fox, who attended Parliamentary meetings dressed like a Colonial to express his support for the rebels, also arguing for the aims of the revolution. Fox went on to become Prime Minister.

John Wilkes was also opposed to the war with the colonies, and a staunch supporter of many of the same values as the colonies, such as religious freedom. In 1774, Wilkes became Lord Mayor of London, a position he used to continue opposing the war with the colonies. In 1779, Wilkes was elected to be Chamberlain of London, a position of great responsibility and power, which he held until his death in 1797.

Throughout Britain, a plethora of influential individuals and groups were sympathetic of the Colonies all during the American Revolutionary War.