What the science establishes is that approximately 10,000 years ago, a person whose genetic profile was typical of the Mesolithic hunter-gatherers of western Europe lived in or near what is now Cheddar, Somerset. His predicted appearance — dark skin, blue or blue-green eyes, dark curly hair — was typical for people of his population at that time and in that part of the world.
What the science does not establish is any particular implication for modern British national identity. The genetic distance between Cheddar Man’s population and any modern British person is enormous — ten thousand years of demographic change, population replacement, and mixing with multiple subsequent migration waves separate them. Claiming Cheddar Man as evidence that “the first Britons were dark-skinned” and drawing contemporary political conclusions from that claim commits the same error as claiming that his dark skin is somehow incompatible with a British identity — both claims project modern identity categories onto a context in which they did not exist.
The meaningful scientific takeaway is about what the Mesolithic evidence does and doesn’t tell us about the long-run history of human presence in the British Isles. That history is one of repeated migration and demographic change, in which the Mesolithic hunter-gatherers were one population among many who have successively inhabited the land. No single ancient population can be identified as the “original” British population in any scientifically meaningful sense, and the genetic ancestry of any modern Briton is a complex blend of contributions from many different source populations across a very long span of time.