The first 'Britons' were an ethnically mixed group From the arrival of the first modern humans - who were hunter-gatherers, following the retreating ice of the Ice Age northwards - to the beginning of recorded history is a period of about 100 centuries, or 400 generations. This is a vast time span, and we know very little about what went on through those years; it is hard even to fully answer the question, 'Who were the early peoples of Britain?', because they have left no accounts of themselves.
'Throughout prehistory there were myriad small-scale societies and many petty 'tribal' identities...' We can, however, say that biologically they were part of the Caucasoid population of Europe. The regional physical stereotypes familiar to us today, a pattern widely thought to result from the post-Roman Anglo-Saxon and Viking invasions - red-headed people in Scotland, small, dark-haired folk in Wales and lanky blondes in southern England - already existed in Roman times. Insofar as they represent reality, they perhaps attest the post-Ice Age peopling of Britain, or the first farmers of 6,000 years ago.
From an early stage, the constraints and opportunities of the varied environments of the islands of Britain encouraged a great regional diversity of culture. Throughout prehistory there were myriad small-scale societies, and many petty 'tribal' identities, typically lasting perhaps no more than a few generations before splitting, merging or becoming obliterated. 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.
The defeated Iron Age tribes of Britain At the end of the Iron Age (roughly the last 700 years BC), we get our first eye-witness accounts of Britain from Greco-Roman authors, not least Julius Caesar who invaded in 55 and 54 BC. These reveal a mosaic of named peoples (Trinovantes, Silures, Cornovii, Selgovae, etc), but there is little sign such groups had any sense of collective identity any more than the islanders of AD 1000 all considered themselves 'Britons'.
'Calling the British Iron Age 'Celtic' is so misleading that it is best abandoned.'
However, there is one thing that the Romans, modern archaeologists and the Iron Age islanders themselves would all agree on: they were not Celts. This was an invention of the 18th century; the name was not used earlier. The idea came from the discovery around 1700 that the non-English island tongues 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.
However, language does not determine ethnicity (that would make the modern islanders 'Germans', since they mostly speak English, classified as a Germanic tongue). And anyway, no one knows how or when the languages that we choose to call 'Celtic', arrived in the archipelago - they were already long established and had diversified into several tongues, when our evidence begins. Certainly, there is no reason to link the coming of 'Celtic' language with any great 'Celtic invasions' from Europe during the Iron Age, because there is no hard evidence to suggest there were any.
Archaeologists widely agree on two things about the British Iron Age: its many regional cultures grew out of the preceding local Bronze Age, and did not derive from waves of continental 'Celtic' invaders. And secondly, calling the British Iron Age 'Celtic' is so misleading that it is best abandoned. Of course, there are important cultural similarities and connections between Britain, Ireland and continental Europe, reflecting intimate contacts and undoubtedly the movement of some people, but the same could be said for many other periods of history.
The things we have labelled 'Celtic' icons - such as hill-forts and art, weapons and jewellery - were more about aristocratic, political, military and religious connections than common ethnicity. (Compare the later cases of medieval Catholic Christianity or European Renaissance culture, or indeed the Hellenistic Greek Mediterranean and the Roman world - all show similar patterns of cultural sharing and emulation among the powerful, across ethnic boundaries.)