I'm looking for any instances where a British soldier voluntarily changed sides mid conflict and fought for the colonies. People who were as infamous as Benedict Arnold but for the British.
Besides the fact that many of the Continental Army officers were trained British officers, including George Washington, Charles Lee, and Horatio Gates, to list a few, nobody as famous(infamous) as Benedict Arnold comes up as switching sides during the conflict.
There were plenty of 'regular' deserters, which chose to join the soon-to-be United States. The German Hessian soldiers, feared by both sides, could often be induced to stay for a stake in property. As could British Soldiers. The congress actually set up a bounty system for awarding land grants to British and Hessian deserters. According to American Agriculture: A Brief History By R. Douglas Hurt
congress lured Hessian soldiers to desertion with offers of 50 acres of land, and tempted British soldiers by providing 50 to 800 acres, depending on rank, as a reward for desertion
The wikipedia entry on Hessian Soldiers does list some individuals:
By early 1778, negotiations for the exchange of prisoners between Washington and the British had begun in earnest. Nicholas Bahner(t), Jacob Strobe, George Geisler, and Conrad Kramm are a few of the Hessian soldiers who deserted the British forces after being returned in exchange for American prisoners of war.
The article later gives the figures that out of the 30,000 Hessians that served in the war,
Approximately 5,000 German troops settled in North America, either the United States or Canada.
. This number seems to have been confirmed in The Hessians and the Other German Auxiliaries of Great Britain in the Revolutionary War, 1884 By Edward Jackson Lowell.
Concerning the number of British deserters during the Revolutionary War, I just found some figures in the book The British Soldier in America: A Social History of Military Life in the Revolutionary Period, by Sylvia R. Frey. Page 72 has the
Official British figures show a total of 3701 desertions in North America and the West Indies
and goes on to state that this doesn't include individuals who returned when pardons were offered, so the initial count would have been higher. So, somewhere between 8000-9000 desertions from the British and Hessian forces facing the Continental Army occurred.
William Alexander, or better known as Lord Stirling was a member of British Parliament who sympathized with the American cause and forsook his Scottish and British lands and his seat in Parliament. He travelled over and joined the American Revolution on the side of the American colonists. He was killed in battle during the American Revolution. But he wasn't an officer on the British side, he was a British government official who had never seen battle or conflict prior to his deserting and coming to America, there upon being made a officer in the American Army.
There is nobody remotely as famous as Benedict Arnold who switched sides to the Americans while the conflict was in progress. The whole business is considered far less significant in British history than it is in American, for obvious reasons.
There was a saboteur, "Jack the Painter" whom I think I might have heard of before I went looking for candidates, but that's the most famous person I can find.
My ancestor was a British officer who changed sides and fought for the Americans. He was given land but refused any reward and chose to stay in the American army and finally settled in southern Alabama. He changed sides on moral bounds after witnessing a women and her daughter being raped by English soldiers. His actions caused problems for his family back in England who he never saw again.