The chief caveat with this topic is that this is antiquity. No academic will provide you with exact percentages because it is disingenuous to pretend we have exact data on all of Roman Britain's demographics. What we have are a collection of written sources, many written outside Britain itself, and a large amount of archaeological data and finds. Synthesising a comprehensive answer that provides you with the exact sort of response you'd like, is simply impossible.

The Roman Empire was large, multi-ethnic, multilingual and multi-religious with an economy that connected it's geographically far apart provinces to one another. There was economic and social reason for someone from one part of the empire to travel to another. Hence it wouldn't be impossible or unlikely to find say, someone from modern day North Africa or the Middle East in Roman Britain - we actually have. It also wouldn't be unusual to find Italian-born people in Iraq and Britain alike. In large cities especially, there were people of multiple ethnic groups and there's more than enough evidence to assume that North African, or Middle Eastern people wouldn't have been unusual in parts of Roman Britain. Neither, for that matter, would be people from other parts of the empire, many large Roman cities throughout Western Europe seem to have had sizeable Greek-speaking communities as well.

There is a good argument to be made that in large parts of Roman Brtain, the South East of England or major cities for instance - that the population could be fairly "Romanised" and Latin-speaking - but the majority of the populace as a whole were rural Celtic speakers. Welsh has quite a lot of Latin influence, suggesting that the ancestors of the Welsh interacted with and were influenced by the Romans. However, a Celtic British majority isn't a Celtic British totality.