Pre Roman invasion in 43 AD, Britons (Brythonic languages etc). 43 AD to roughly 400 AD, Romano Britons and various combinations and graduations thereof. Roughly 300 or 400 onwards, Saxons, Angles, Jutes start to turn up in these isles. The Romans are generally considered to have buggered off around 400ish.

The largely Angle/Saxon incursion was not a smooth considered invasion but more of a spreading out. Bear in mind that travelling by sea was generally easier than by land at the time. Heavily wooded hills are a pain to cross compared to sea with a decent wind. So the sea back then was not a barrier but actually helpful.

300?-1066 - What will become the UK etc is an amalgam of tiddly kingdoms and whatnot. What becomes England is Wessex, Dumnonia (Devon and Cornwall - a bit Welsh - west wales), Kent, Essex, Sussex, Norfolk, Mercia, Deira and quite a lot more. There was the Danelaw and Dalegelt business with a bunch of homicidal viking folk. York's, and hence New York's names are derived from Jorvik which was the Danish name for the original town. The Romans called the place Eboracum.

Then the Normans turn up in 1066. lol

> York's, and hence New York's names are derived from Jorvik

There is a step between Jorvik and the Latin Eboracum: Eoforvic.

The point is that the concept of "indigenous" people is extremely limited, how large does a piece of virgin land which was never stolen need to be, for there to be nobody there but the indigenous people? Is there a length of time which makes a people indigenous after conquering land once?

The Mesolithic population represented by Cheddar man (https://en.wikipedia.org/wiki/Cheddar_Man) may be the closest we know to the first inhabitants of the British Isles after the last ice age (10k yrs ago). There is evidence of habitation before the last ice age but no human remains have been found. Based on DNA analysis Cheddar man is thought to have had dark skin, blue eyes, and despite his name, been lactose intolerant:

https://www.bbc.com/news/science-environment-42939192