Just as the American colonists were divided in their loyalties to the Crown, the British people were divided in their support of the war. After all, they had recently concluded a seven-year war against the French (1754-63), American privateers were raiding British merchant ships and disrupting trade, and the British military, while well trained, was spread thin across the empire. The British knew they would be fighting a long and costly conflict over 3000 miles away, and they were still paying for the last one.

The American colonies only supplied 1% of British GDP; the real value lay in the Caribbean colonies, which were far more profitable.  

“At the most moderate of computations, this war will cost Great Britain 10 millins of treasure and 20,000 lives. What then, in the name of wonder is the object of war? Are we to throw away so much treasure and so many lives to gain a point which, when gained, is not worth 1% on our money?” – The Public Advertiser

Voices in Parliament argued for more autonomy for the colonies, igniting debate. Many British naval and army officers refused to fight against their fellow Englishmen, either resigning their commissions or requesting a transfer to another post. The Lord Mayor of London asked King George to reconsider his position. Unfortunately, the stronger voice came from Lord George Germain, the British secretary of state for the American colonies, who was determined to squash the “rebellion,” fearing that Britain’s Catholic enemies France and Spain would step in to support the rebels. And, he argued, if the American colonies fell, Canada and India might soon follow.

https://allthingsliberty.com/2017/01/those-who-could-not-serve/

Ken Burns, The American Revolution