Because there are no direct written accounts of pre-Roman life in Britain, anything we know about life in those days comes from Roman or other classical sources, or from archaeological digs.
Rosemary Sutcliffe wrote a number of 'Celtic' books for children - she was a good writer and I'd recommend giving her a go - here's a full list:
http://en.wikipedia.org/wiki/Rosemary_Sutcliff
She tried to base her stories on actual evidence rather than flights of pure fantasy.
In addition, you could try any reprinted stories from The Mabinogion. These are supposedly ancestral Welsh stories that survived long enough to be written down in the Middle Ages, and so could give insights into how pre-Roman people lived. However there is some debate as to their origin.
I am sure you will find less well-known writers who are in the genre of Celtic fantasy / hippy dippy magic-making arthurian circle dancers but while they may write a good yarn most of what they are saying is pure fiction and does not reflect accurately on pre-Roman life in Britain.