Who are these ‘indigenous Britons’ I’ve heard so much about?

As far as I can see, there wouldn't be a Britain if it weren't for immigration. The first people to immigrate, to what is now Britain, were the Celts, who came from southern Europe. Other groups who immigrated to Britain were the Anglo-Saxons, Vikings, Normans (French; and Danes who were exiled from Britain), Jewish people, people from Africa, and people from the near, middle and far east; each group has brought their own culture to Britain. The official language of England, for six hundred years, was French: Britain has been built on immigration, and multiculturalism, even before there were words for them.