No, his name was not Cymbeline (that's a Shakespeare character based on the actual King of the Britons). Although Britons hadn't learnt to write until the 6-7th century the Greeks and Romans first wrote his name for him. Aaahh
So you see although he freely caught people and sold them into slavery (to the Romans) he was unable to write/discuss the ethics of this practice of his.
Aristotle debated the ETHICS of "slavery" and did not practice a slave trade (neither did the Greeks - they never sold slaves, they kept in servitude those who lost wars against them, that's all.)
The other contextual factor with Aristotle that academics have only recently come to appreciate is that when he used the word for "slave" it was the word for what we now ascribe to someone who needs to be "taught", to "learn", unschooled, a child, etc so if you reread his ethics with that in mind, it all makes more sense.
The ancient Greeks had many words to describe slaves which need to be placed in context to avoid ambiguity. In Homer, Hesiod and Theognis of Megara, the slave was called δμώς / de-m-ō-s.[5] The term has a general meaning but refers particularly to war prisoners taken as booty,[6] in other words, property. During the classical period, the Greeks frequently used ἀνδράποδον / andrapodon,[7] literally, "one with the feet of a man", as opposed to τετράποδον / tetrapodon, "quadruped", or livestock.[8] The most common word is / doulosδοῦλος / doulos,[9] an earlier form of which appears in Mycenaean inscriptions as do-e-ro,[10] used in opposition to "free man" (ἐλεύθερος / eleútheros). The verb δουλεὐω (which survives in modern Greek, meaning work) can be used metaphorically for other forms of dominion, as of one city over another or parents over their children.[11] Finally, the term οἰκέτης / oiketēs was used, meaning "one who lives in house", referring to household servants.
wiki
So it's accepted now that Aristotle was debating (as a biologist) that some people are naturally gifted in learning (intelligent) and others less so and needed to be instructed more.
Opposing even the practice of keeping servants or POWs as workers were the great
Stoics who condemned all thoughts of men being unequal - the precursors of Christianity.
But the fact remains, when the Classical Greeks had long
debated the ethics of slavery and given up all such ideas - the Ancient Britons were happily trading their own people as SLAVES along with hounds etc...
[ So the question now becomes: are the Britons the ones who
hold the record for the longest known practice of the slave trade, only abolishing it recently? Some 2,000 years of recorded history? ]
Poor Africa ...