Did English common law even recognize slavery as a status in the 16th century? By then the English are involved in the slave trade, but perhaps not taking any of those captured to England. The Spanish and Portuguese had a tradition going back to Roman law about slavery, reaffirmed by the church in the 1400s...one of the legitimate categories justifying slavery was “taken prisoner in a just war”—so any slaves purchased from dealers at trading posts on the African coast could be classified as prisoners of war taken in a just war. Somehow all the wars that resulted in captives were just wars.
I am not a common lawyer.
But the institution was outlawed in England in 1772 by the Somersett case which was based on common law, not a statute.
However, it had been accepted for several centuries there previously.