Ok, I think I owe some answers here.
how long have the Celts lived in Britain?
Since 21 June 1792.
It was then that a group of London ‘bards’ staged an entirely invented ceremony on Primrose Hill in London, involving a stone circle made from pebbles,
and claimed they were reviving a ritual that stretched back to the ancient Celtic nation and its Druids.
Prior to this, there is no record of the word ‘Celt’ having been used to describe the pre-Roman inhabitants of Britain or Ireland and it was certainly never
a term they used to describe themselves.
The word ‘Celt’ was coined by the Greek historian Herodotus in 450 BC when he described the peoples of the headwaters of the Danube north of the
Alps.
The Roman name for such people was Galli (‘chicken people’) and they called the inhabitants of the British Isles Britanni, never Celts.
The use of the term ‘Celt’ in English dates from the seventeenth century.
A Welsh linguist living in Oxford called Edward Lluyd noted the similarities between the languages spoken in Ireland, Scotland, Wales, Cornwall and
Brittany. He called these languages ‘Celtic’ and the name stuck. The word ‘Celtic’ has also been used to describe the curly-wurly
style of design found in Irish gift-shops. There is no evidence to suggest that this was produced by an ethnically homogeneous group of people.
Most historians believe the language and culture we call ‘Celtic’ spread by contact not invasion. People ‘became’ Celtic by adopting the architecture,
fashions and ways of speaking because they were useful or attractive, not because they belonged to the same ethnic group.
The romantic notion of a Celtic Empire of horse-loving master craftsmen, wise old Druids, harp-strumming poets, and fierce bearded warriors is the
product of the Celtic Revival that started in the late eighteenth century.
It has more to do with modern Irish, Welsh and Scottish nationalism than with any historical reality.