Here's something else to consider.
It's now thought to be a myth that 'invasions' like the Anglo-Saxons, the Danes, Normans etc replaced or drove out the indiginous population.
They didn't, well not for most of the population....the bigwig landowners might have left/been killed/been replaced, but most people stayed where they were. They simply had different rulers - same old shit.
So the Welsh are not neccessarily the remnants of the pure-bred Celt, any more than the English are.
What changed in the invasions was the culture/governance/technology, not the people.
Maxhardcore