You have left out some important details.
In practical terms, only slaves below the age of six were freed in the colonies. Former slaves over the age of six were redesignated as "apprentices", and their servitude was abolished in two stages: the first set of apprenticeships came to an end on 1 August 1838, while the final apprenticeships were scheduled to cease on 1 August 1840. The Act specifically excluded "the Territories in the Possession of the East India Company, or to the Island of Ceylon, or to the Island of Saint Helena." The exceptions were eliminated in 1843.[17]It's not like, "Hey Presto" and it was done.Payments to slave owners
The Act provided for payments to slave-owners. The amount of money to be spent on the payments was set at "the Sum of Twenty Million Pounds Sterling".[18] Under the terms of the Act, the British government raised £20 million (£16.5 billion in 2013 pounds, when calculated as wage values)[19] to pay out for the loss of the slaves as business assets to the registered owners of the freed slaves. In 1833, £20 million amounted to 40% of the Treasury's annual income[20] or approximately 5% of the British GDP[21] (5% of the British GDP in 2016 was around £100 billion).[22] To finance the payments, the British government had to take on a £15 million loan, finalised on 3 August 1835, with banker Nathan Mayer Rothschild and his brother-in-law Moses Montefiore. The money was not paid back until 2015.
My point is that England was among the first, if not the first, European nation to abolish slavery. They did so before we did, and without a war. Had they defeated us during the Revolution, then due to the Philipsburg declaration slavery would have ended here with our defeat.
If we are going to play the ever popular moralizing over slavery game then lets quit pretending that we have the edge. Britain abolished the Atlantic slave trade the very same year that our Constitution abolished it. And they abolished slavery in their colonies before we abolished it here. Moreover Britain’s navy put a whole lot more energy into shutting down the slave trade than we did. Nothing more complicated than that, other than that falling into the moralizing trap in the first place is a fool’s game.