Well, the first 'indigenous' people were those using the land bridge to travel to the unpopulated land that is now Britian after the last ice age.
These disappeared with the arrival of the Neolithic people. At first this was assumed to be a cultural replacement, but evidence now shows they were a new wave of people, then there was the Beaker culture, also now known to be new arrivals not just culture. Following on we have people moving quite freely between Britain and Europe, even after the land bridge was destroyed by rising sea level rising and a tsunami. After that there were 'Roman' soldiers from all over the known world (then) who came and settled, then Germanic invaders, ditto Scandinavians, and since then a smattering of Normans, Hugeonots, Irish and Scots etc etc etc
Exactly who are the 'indigenous' Brits?