Fascinating stuff Paul. Not questioning your facts, but can you give any sources? I'd like to read a bit more around the subject.
I first got interested in this whole area when I got to wondering "what happened to the 'Ancient Britons' after the so-called Anglo-Saxon invasions?". What we used to be told at school was that those who weren't killed, were driven to the extremities of the west and south-west (i.e. Wales and Cornwall).
However, the more I read about the subject, the more doubt there seemed to be about that.
Trying to answer that question took me down all sorts of interesting avenues and by-ways, which I can't go into here. But I got the impression that the conclusion of archeologists and historians nowadays was that there was no major "ethnic cleansing" of the then indigenous people. True there would have been battles and killings, but the survivors, by and large, remained where they were, and the invaders were gradually assimilated.
And the same was true to a varying degree for the later Viking and Norman invasions.
So the answer to the question: "what happened to the 'Ancient Britons' was: Look around you: their descendants are still here, mixed in with the genes of North and West Germanic peoples, and those of the Normans.
However, this does not quite tie in with your statement:
(Although obviously the Belgic tribes came a lot later, starting before the first Roman invasion I believe. However, I think they were confined to some distinct areas in the south-east, and may not have been all that numerous.
Edit: Actually, where I live, in the southern part of Oxfordshire, and a bit further south, was dominated by the Belgic Atrebates: https://en.wikipedia.org/wiki/Atrebates ).