Which of the many cultures of Great Britain have defined what it means to be British?

People cling to the identity of Britishness without questioning what that is. First of all we were a Megalithic people, then the Celts arrived, that worked fine for a while. Romans, Angles, Saxons, Vikings, Normans added to that mix. When people complain about the British identity or British way being under threat are they simply opposed to including people of a different skin tone? We have always been multicultural and always will be. What is it that makes Britons cling to a particular definition of Britishness and what is that definition? We now have 3rd, 4th and 5th generation Carribean, African, Indian, Chinese, Russian, Polish, Italian, Armenian, Greek, Pakisatani British people who pay the same dues and have the same rights as older ethnic British strains. We are a mongerel Nation, we are One. What makes you British? People say they would die for it but can you be big enough to live for it? What does it mean to be British?