This was brought up by seeing Mary Beard say that there is plenty of evidence of an ethnically diverse Roman Britain https://twitter.com/wmarybeard/status/889925415032299520 And in the comment thread she even goes on to say that there might have been Sub-Saharan Africans.
Now, while I recognize race is a social construct, the importance given to different phenotypical differences is not transhistoric, etc. I am very curious whether the Roman Empire extended in such a way as to allow for Sub-Saharan Africans to move through out it and eventually end up in Roman Britain. Along with what other ethnic groups might have existed in Roman Britain at the time (brought along by the Romans).
It's very likely that there were. The various historians responding to Paul Joseph Watson's staggeringly ignorant statement are pretty much giving you the evidence: there is epigraphic, iconographic and archaeological evidence for the presence in Roman Britain of people from a wide range of ethnicities. This may well have included Sub-Saharan, or, at the very least, East Africans.
Modern historians of the Roman economy regard the Roman Empire as one of the first world systems - an interconnected geographical area in which trade, travel, goods, ideas and people flowed in all directions. One of the groups swept up in this world system as part of the Empire was the very mixed population of Egypt, which had long consisted of both Mediterranean peoples from Lower Egypt and Nubians from Sudan. The Nile provided ready access to Egypt for the peoples of inland East Africa, and both migration and war between these regions had been going on intermittently for millennia when the Romans appeared on the scene.
Apart from migration related to trade and economic opportunity, the Roman Empire also facilitated ethnic diversity through its practice of garrisoning parts of the empire with military units raised elsewhere. We know that there were Syrians serving in Egypt and Batavians in Dacia. We know that commanders from Libya could become Emperors in Rome. We also know from inscriptions that there were Syrians serving in Roman Britain. It seems all too likely that, even if trade hadn't brought any Egyptians to Roman Britain, the army would have.
In addition to the Imperial province of Egypt, the Romans also had contacts with regions further afield. From the east coast of Egypt, a trade network extended down the Red Sea, around the Arabian peninsula, and on to India. Intensive trade was carried out with the Ethiopian kingdoms (notably Aksum) and with India; Roman naval bases were established on islands at the far end of the Red Sea. While much of this trade network would have been run through relay between different carriers, it is likely that the Ethiopians themselves, or even people from India, would have made it into the Empire to get more closely acquainted with their trading partners.
In short, it's perfectly plausible that some of the people inhabiting Roman Britain were from Sub-Saharan Africa, Ethiopia, and India (not to mention Arabia, modern day Ukraine, Morocco, etc).