I doubt that 50% of the population is non indigenous. It may be true that immigration has changed the mix in urban areas, but in the countryside, the majority are locals who have their roots there for hundreds of years.
What is an idigenous Briton? The Celts originally came from Northern Spain and Britanny, The Anglo Saxons and Vikings came from North Western Europe and the Normans came from Northern France, although they were actually descended from the Danes themselves. William the Conquerer had Viking Ancestors.
