The earliest recorded names of individuals and tribes in Britain come from J Caesar.

He mentions:

celtictribesengland More is on record by the time of the Claudian invasion in 43 BCE and an overall view of the main tribal groupings can be reconstructed. This view is undoubtedly incomplete because each tribal group or "clan" will have been divided into sub groups or "septs" about which almost nothing is known.
Ptolemy maps Great Britain and Ireland and Tacitus writes about the north of England and Scotland.

scottribesTheir picture for the first century CE is fairly convincing but certainly incomplete. Very little is known beyond the names of the people which all seem to be Celtic. In addition to tribes and settlements, Ptolemy also names a few rivers and some coastal features.

*tri|novantes very|vigorous
probably (I)ceni magni
people of Segontios (the victorius)
*an|caleti very|hard men
*bibro beaver (men of the beaver)
*cad-ti excellent (the excellent men)