If the historian Tacitus simply provides too much of a Roman perspective and/or is too late in history (1st century AD), then archaeological evidence is likely the best source (as DVK previously commented).
The people of Iron Age Britain
Demography The Roman historian described the Britons as being
descended from people who had arrived from the continent, comparing
the Caledonians (in modern-day Scotland) to their Germanic neighbours;
the Silures of Southern Wales to Iberian settlers; and the inhabitants
of Southeast Britannia to Gaulish tribes. This migrationist view long
informed later views of the origins of the British Iron Age and,
indeed, the making of the modern nations. Linguistic evidence inferred
from the surviving Celtic languages in Northern and Western Great
Britain at first appeared to support this idea, and the changes in
material culture which archaeologists observed during later prehistory
were routinely ascribed to a new wave of invaders.
Other Resources: