Short answers, pulled from the story.
Celtic Britons were the Celtic people who inhabited Great Britain from at least the British Iron Age until the High Middle Ages. They spoke Common Brittonic and eventually diverged into the Welsh, Cornish, and Bretons, among others. The earliest written evidence for them comes from Greco-Roman writers dating to the Iron Age.
Celtic Britons spoke Common Brittonic, an Insular Celtic language derived from Proto-Celtic, which arrived in the British Isles from the continent between the 10th and 7th century BC. Common Brittonic developed into Welsh, Cornish, Cumbric, and Breton. Welsh and Breton survive today; Cumbric and Pictish died out in the 12th century, and Cornish has been revived since the 20th century.
The Roman Empire invaded Britain in 43 AD and by 84 AD had decisively conquered the south, establishing the province of Britannia. Hadrian's Wall, built in 122 AD, marked the northern limit of Roman control; Brittonic tribes such as the Caledonians and Picts north of the wall remained unconquered. The Romans departed around 410 AD, after which Anglo-Saxon settlement began.
Following the end of Roman rule, Britons established colonies in Brittany in what is now France, the Channel Islands, and a settlement called Britonia in northwestern Spain, which first appears in the historical record at the First Council of Lugo in 569 AD. These migrations gave rise to the Breton language, still spoken today, and left place-name traces across Galicia and Asturias.
A 2016 study examined Iron Age Britons buried around 100 BC at Linton and Hinxton in Cambridgeshire and found their genetic profiles typical of Northwest European populations but markedly different from later Anglo-Saxon samples. A 2018 study found that native Britons of Roman Britain were genetically close to modern Welsh people, suggesting continuity from Iron Age Britain to the present day, while Anglo-Saxon settlers brought a distinct and lasting genetic change.
The P-Celtic ethnonym has been reconstructed as Pritani, likely meaning "people of the forms" or "shapely people". The ancient Greeks called the islanders Pretanoi or Bretanoi. The scholar John Rhys introduced Brython into English usage in 1884 to refer specifically to P-Celtic speakers of Great Britain, and the term Brittonic languages was first recorded in 1923 according to the Oxford English Dictionary.