who were then invaded and ruled by the Vikings in large swaths and then by the Normans and so on.

DNA analysis of early medieval graves show there were people of West African and Middle Eastern ancestry, and they were not slaves. "Britain" had long established trade with faraway lands even before the Romans and it is not impossible that some of these traders settled there.

The Romans had slaves. They are still digging out at Sutton Hoo. I'm a faithful fan of Time Team

First of all, Sutton Hoo is not a Roman site. It is Anglo Saxon.

I would also point out that the Roman institution of slavery was not what came later during the period of European Colonialism starting in the 16th century as Roman slavery was not based solely on ethnicity, race or geographic origins.

In Britain, the Celtic tribes or even individuals who cooperated with the Romans after the invasion were not enslaved any many later became full Roman citizens after incorporating Roman customs and religion (even if still retaining some of their old ways).

If you are a fan of Time Team, you’ve probably seen excavations of late Roman villas what were owned by British-Romanos, i.e. those born in Britania who were not born in Rome or even originally Roman Citizens.

Same in other parts of the Empire. For instance, many Jews in Judea and in places like Alexandria with large Jewish populations, were Roman citizens either through birthright or manumission.

Manumission could be granted to Roman slaves of any race or origin or even religion for various reasons but mostly because they had attained trusted positions, demonstrated loyalty and even an education.

In Roman world there were slaves that ran and with a good degree of autonomy, businesses on behalf of their masters and granting a slave manumission could be beneficial to their former owner as well