Colonisation by us humans began around 14,000 years ago, once things had warmed up enough after the Ice Age and the tundra had thawed out sufficiently to allow tree cover to return.
If there were hominins living in Britain before the latest Ice Age, they spent the winter in south western Europe, just like some of their wealthier modern counterparts do today. But their “winter” was to last several thousand years. These Iberian shelterers have been identified as Celts. As soon as the climate improved, some of them took to their ships and colonised the west of Britain. Genetic markers show that these early settlers have had a big influence on the current population. Meanwhile the Gaels (possibly originating in Asia) moved through central into western Europe, bringing other “celtic” languages and cultures with them.