“Slavery did not exist under English law”
Yes, slavery existed under English law during the colonial period and was eventually eradicated by Wilberforce.
How do you think the slaves got to North America in the first place? Swimming?
I mean as the law of the land. Dred Scot unsuccessfully appealed to the rule explicated by Lord Mansfield that any slave brought to the free soil of England could claim his freedom as a subject of the King.