Slavery might have lasted longer, or turned into a kind of peonage.
Slaves were needed to work the cotton plantations in the south, also the sugar plantations and tobacco plantations. Those were major cash crops. If they had maintained control of India as well, then Britain would have had a near monopoly on cotton production. You have to remember that cotton was the wonder fiber of the 18th and 19th centuries, and the ability to produce cotton cloth cheaply revolutionized the textile industry.
England was a pioneer in the textile industry. If the U.S. had lost the revolution, cotton would have flowed steadily from Charleston and Savannah, and (I expect eventually) New Orleans, to the textile mills in Manchester and Leeds. I doubt that the British would have wanted to interfere with that nice, steady flow.
Then, of course, there's tobacco, and sugar, also very labor intensive crops, but very lucrative ones. It is possible that Parliament might have abolished slavery by promoting slaves to serfs. (Not much of a promotion.)

I might also mention one of the lowest moments in Pittsburgh history. Sir Jeffrey Amherst, the Royal Governor of North America wrote to Colonel Henri Bouquet, a Swiss mercenery who commanded Fort Pitt, reccomending that he provide the local Indians with blankets and various items of clothing that had been used by people who had died of smallpox, to spread the disease among the Native Americans. To paraphrase Sir Winston Churchill, "This was NOT England's finest hour."