Some years ago I heard a interview with some Englishman.,perhaps from the Midlands. He and a number of others in his village had their DNA matched to that recovered from the bones of a excavated grave dated back several thousand years. At first he was quite excited to learn of the connection to folks who had lived there so long ago. Then he realized it meant none of the intervening ancestors had the ambition to wander off in search of anything new
Yes indeed. The background baseline is that most people in Britain are about 85% genetically identical to the pre farming population. Talking about saxons or whatever is talking primarily about culture not genetics. Although there is questionable evidence of a genetic difference between the west and the east even that far back. I take the view that, from the absence of celtic names in the south and east and the names the Romans found for the tribes there, there was an extensive presence of a similar culture between the east of england and the opposite German Ocean coast. Hence, as has been said above, the 'Saxons' (speaking myself as an East Anglian) were simply the last suspects.
Vortigern was not the British leader and the Saxons were here and more arriving all along the south and east coast so him not inviting Hengist, Horsa etc. would have made no real difference. Perhaps it gave a foothold to a particular group of 'Saxons' (Jutes) who had trouble finding a foothold where Saxons and Angles were already in place. Hence the Jutish settling into Kent and Hampshire.
I can imagine the sailing instructions for Jutish sailors. 'Head into the setting sun, when you find land on both sides turn right.'
There is a hypothesis that says that the British resistance to the Saxons was a class affair whereby the latinised British aristocracy and middle classes were trying to maintain their hold on the traditional unlatinised peasants. In support of this is the settlement of Brittany by the British allegedly fleeing from the Saxons. The written tales suggest that this was the aristocracy moving out with their households but we certainly know the peasantry stayed in place and embraced the (familiar?) Saxon culture. In the west the peasantry were unfamiliar with Saxon culture and the aristocracy was more traditional. Hence the west was more difficult for the Saxons to take being ungermanic hence Cumbria, Cornwall and Wales retained their Brythonic cultures and languages. The first losing it in the medieval period, Cornwall in the 17/18th century and Wales still has it.
I don't know about S4C with a 0 viewer status but, years ago in Manchester, my (then) little children preferred to watch SuperTed in Welsh and had a curious affection for Pobol y Cwm.