The sole source for the existence and location of these tribes are Roman writers who visited Britain.
One of the best observers of the tribes of Celtic Britain was Tacitus who wrote on historical events in Britain. Another was a Roman geographer called Ptolemy who wrote a description of Britain, listing the names of the many British tribes.
However, we must treat these writings with care as they are as much propaganda as impartial historical accounts. Furthermore, the Roman writers sometimes misunderstood British events or even the names of the tribes. This is especially true of their descriptions of tribes in the north where Roman knowledge was even more limited. Indeed, precise information about where these tribes lived is entirely absent from the contemporary Roman records.

The British Isles and First Century AD tribal regions.
Key Tribal Regions