The Celts, whether in mainland Europe or Britain,
were not a literate people. Let's describe them as
aliterate, rather than illiterate.
They amply compensated by being highly skilled
in the arts and crafts. The outstanding craftsmanship
of e.g. the Desborough Mirror and the Battersea Shield
are good examples. The Romans were greatly
impressed by the skill of these British 'barbarians',
learned immensely from them, and donated literacy
to them in return, and that was the end of the oral
tradition of the Celts.