The first thing you need to understand here is that slavery wasn't fully illegal in England. William the Conqueror had passed a law that outlawed trading slaves overseas but the slavery trade in Norman England largely ended because of local and religious efforts without explicit national prohibition ("The Oxford History of the Laws of England Volume II: 871-1216", John Hudson (2012). It had continued after the Roman departure until changing attitudes curtailed it's use, and agriculture in the Medieval Period improved so there would have been less motive for it when production got better anyway. Later on common law interpretation had that there could be no slaves, but this was overturned during the slave trade because there was no decisive act to establish it. It's also worth noting than tenants, bound apprentices and indentured servants had less freedoms than a free tradesman, so there were forms of semi-forced or controlling labour that existed otherwise. Work could often be quite liberal in pre-industrial society, with less of a sense of regular hours, frequent holidays and often mobility between jobs or doing multiple different jobs. Employers didn't necessarily expect a loyal, bound employee. But the restricted forms could be fairly abusable, given the power dynamics and lack of strong labour protections.

Furthermore there was slavery in the 16th century in one or two manifestations, depending on how you interpret the later one. This trade involved Africans and very much was the predecessor of the Atlantic Slave Trade. Sir John Hawkins had multiple voyage in the late 16th century where he gained and sold African slaves. The second, somewhat more murky and debatable one is the presence of African servants in late 16th century England. Some scholars see them as pseudo-slaves given they were often take from Portugese slave traders, which made them indebted and reliant on their "saviours", while others emphasises that there is largely evidence of them being paid wages and protected from those who would enslave them (compare Tamara Lewis' "'Like Devils out of Hell': Reassessing the African Presence in Early Modern England" (2016) with Miranda Kaufman's "Black Tudors: The Untold Story" (2017) for more negative and more positive angles respectively). There is limited evidence as to whether these people were subject to the same or different labour treatment, while many appear. I did an answer on this latter aspect, and other aspect of minority communities in London and the time, in the 1482-1609 floating feature from the "Story of Humankind" drive if you are interested in that bit.

Lastly you must consider that while England had fairly minimal involvement in it slavery was a major thing for the Hapsburg Spanish and Ottoman Empires at this time, who traded also with Barbary (North African Berber) slavers and there auctions in parts of Italy. So the adoption of slavery in Virginia from 1619 was not a very unusual thing for their time, even if it was a historical milestone in England's development towards becoming the biggest slaver of all in the 18th century. Racial equality was not an idea of the the time by any means, and while racism was promoted and formalised in news ways as slavery was implement and needed to be justified, the early 17th century was perfectly capable of being racist. In a relatively localised self-contained society, anything "foreign" or "strange' was subject to potential xenophobia. Slavery's introduction in imperial England was also justified religiously, under a theory of Christian rights where Africans were deemed inherently and uniformly heathenous and not subject to the same rights. Slavery might even be means to acquire converts, by transporting them to Christian countries. Furthermore, the Bible also generally mentions and seems to condone slavery Robin Blackburn "The Making of New World Slavery: From the Baroque to the Modern, 1492-1800" (1997) In a hierarchical society, this made perfect sense.