The topic of early languages in Britain is one that's always under review and with good reason. The earliest known occupation of Britain dates back to 900.000 years BCE, although this was mainly Neanderthals. Humans finally arrived around 500.000 BCE, as evidenced by fossils in Sussex, during the period when Britain wasn't an island but connected to the Continent by a chalk ridge between South East England and northern France called the Weald-Artois Anticline.
There's evidence from bones and flint tools found in coastal deposits near Happisburgh in Norfolk and Pakefield in Suffolk that a species of Homo was present in what is now Britain at least 814,000 years ago. At this time, Southern and Eastern Britain were linked to continental Europe by a wide land bridge (Doggerland) allowing humans to move freely. The species itself lived before the ancestors of Neanderthals split from the ancestors of Homo sapiens 600,000 years ago. The current position of the English Channel was a large river flowing westwards and fed by tributaries that later became the Thames and Seine. Reconstructing this ancient environment has provided clues to the route first visitors took to arrive at what was then a peninsula of the Eurasian continent.
Britain was unoccupied by humans between 180,000 and 60,000 years ago, when Neanderthals returned. By 40,000 years ago they had become extinct and modern humans had reached Britain. But even their occupations were brief and intermittent due to a climate which swung between low temperatures with a tundra habitat and severe ice ages which made Britain uninhabitable for long periods.
Written linguistic evidence only exists post-Roman occupation and opinion is divided on what preceded it. However, what is fairly well established is that we all originated in India or Africa from a small number of tribes and probably had rudimentary language skills but as soon as the great migrations from Africa got underway, tribal languages diversified.
Today the most widely spoken language is English, which has become the de facto international language.
It's also worth considering that language and religion are the two main causes of wars and genocide.