Latin would be the earliest British language however you must remember that what we now know as Britain has not always been British (the Romans used to own it).
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The Romans conquered Britain between AD 43 until ca. AD 410 and during this period introduced Latin. However Britain has a culture going back thousands of years before the Roman occupation (during which period Stone Henge was built) - DNA analysis has shown that modern humans arrived in Britain at least 25,000 years ago.
These ancient British people had a language and culture of their own but sadly none of the pre-Roman inhabitants of Britain have any known, surviving, written language. Generically they are called Celts and there is a small amount of linguistic evidence of Celtic language that comes down to us from British river and hill names.
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The Celtic language was spoken by the ancients in the regon, but the language spoken in Britain is not known. It was not recorded.
The word is descended from the ancient Celtic language, which is a member of the Brythonic group of languages spoken natively in Wales, along the Welsh boarder and into local parts of England
No one alive today knows how ancient Egyptian was spoken.
Latin.
They saved ancient Greek and Roman writings.