Before Romans times, “Britain” was just a geographical entity with no political meaning and no single cultural identity.
The gene pool of the island has changed slowly and the idea of large-scale migrations has been widely discredited. The island has always consisted of multiple cultural groups and identities, many of which looked across the seas for their closest connections - they did not necessarily connect naturally with their fellow islanders. Genetic studies are revealing much more of this island's past and it makes fascinating reading.
From the arrival of the first modern humans - who were hunter-gatherers, following the retreating ice northwards - to the beginning of recorded history is a period of about 100 centuries, or 400 generations; a vast time span we know very little about.
Biologically these people were part of the Caucasoid population of Europe. The accepted regional physical stereotypes - red-headed people in Scotland, small, dark-haired folk in Wales and lanky blondes in southern England - existed in Roman times.
Different environments encouraged a great regional diversity of culture that consisted of small-scale societies, petty 'tribal' identities, typically lasting perhaps no more than a few generations before splitting, merging or disappearing. These groups were in contact and conflict with their neighbours, and sometimes with more distant groups - the appearance of exotic imported objects attest exchanges, alliance and kinship links, and wars.
At the end of the Iron Age (roughly the last 700 years BC), the first eye-witness accounts of Britain appear. Greco-Roman authors like Julius Caesar reveal a mosaic of named peoples (Trinovantes, Silures, Cornovii, Selgovae, and many more), but there is little sign such groups had a sense of collective identity.
Romans, modern archaeologists and the Iron Age islanders themselves all agree that they were not Celts. This was an 18th century invention; the name was not used earlier. Around 1700 the non-English island tongues were found to relate to that of the ancient continental Gauls, who really were called Celts. This ancient continental ethnic label was applied to the wider family of languages. But 'Celtic' was soon extended to describe insular monuments, art, culture and peoples, ancient and modern: island 'Celtic' identity was born, like Britishness, in the 18th century.
Dr Simon James is Senior Lecturer in Archaeology at the University of Leicester. He specialises in Iron Age and Roman archaeology, Celtic ethnicity and the archaeology of violence and warfare. I found an very interesting article of his, quite long, and the above is my attempt to whittle out the main points so that I will remember them. You may wish to read the entire article: http://www.bbc.co.uk/history/ancient/british_prehistory/peoples_01.shtml#top