It is interesting to note that the British enticed slaves to escape their masters and fight for the British, promising them freedom in return. (Dunmore’s Proclamation)
Then, they completely reneged on that promise after they used them, badly, as cannon fodder and such, and used loopholes to get them enslaved again, sending them to plantations in the Caribbean, etc.
This is not to diminish the actions of England in helping to stamp out slavery, who led the way in the West.
The Dunmore’s Proclamation was a bill of attainder which stripped patriots of their property including slaves. The slaves themselves were given a narrow set of choices and as you say, the Brits reneged on their deal.
Then, they completely reneged on that promise after they used them, badly, as cannon fodder and such, and used loopholes to get them enslaved again, sending them to plantations in the Caribbean, etc.
This is not to diminish the actions of England in helping to stamp out slavery, who led the way in the West.
I often point out the similarities between the Revolutionary War, and the Civil War. Most of the "Union uber alles" types do not like these comparisons because it puts their side in a bad light.
But in this case, at least the Union kept their agreements with the slaves.