As I pointed out before, slavery in England was not legal @ the time of the American Revolution. See
Slavery in Britain - Wikipedia
"
Slavery in Great Britain existed and was recognised
from before the Roman occupation until the 12th century, when chattel slavery disappeared, at least for a time, after the Norman Conquest. Former slaves merged into the larger body of serfs in Britain and no longer were recognized separately in law or custom.[1][2]
"From the 17th century into the 19th century, transportation to the colonies as a criminal or an indentured servant served as punishment for both serious and petty crimes, or for simply being poor and viewed as an 'undesirable', facilitated by the Transportation Act of 1717.[3] During the same period, workhouses employed people whose poverty left them no other alternative than to work under forced labour conditions."
(My emphasis - more @ the URL)
As Colonial/US law largely derived from British law, that means that slavery was not legal in the colonies nor nascent US either, until the US states began to incorporate slavery into their lawmaking. (Even though there was some discussion of slavery in the US Constitution, & policy on slavery was set @ the national level.)