Interesting stuff but I'm not sure I believe it, let me explain why
A spaniard breeding with a native scot and hundreds of years later his features are still around? Even when a black african mixes with a white, in just a few generations the "blackness" is almost completely gone. They might have a little puffy hair but in general the later generations just look white. Now, in my case, my family has had the swarthy look for hundreds of years. I think this implies that BOTH sets parents carry the DNA. I think it's that good old fashioned Britannia DNA (my theory) Before the celts, before the saxons etc and for this reason the swarthy look has never been "bred out".
I agree with your general point about most peoples ancestors being Celtic or pre Celtic to the British Isles/Ireland.
There seems to be a fashion, and this is especially true in the USA, to make any Irish/Scots ancestors somehow ethnically
different to the English. This is why there are so many myths floating around with regard to 'Basque', ' the Spanish Armarda' and even 'Egyptian' ancestry. None of it actually proven to be true. The Irish and Scots are not much different ethnically to the rest of the North Western European population.
The need to make their ancestors
different to the English is probably more tied up with political ideas about identity, rather than based in fact. During the 18th and 19th centuries there was a tendency to exaggerate and romanticise 'Celt' and 'Anglo-Saxon' heritage and history. Before that time it was hardly mentioned in history books, novels and literature at all.
Terms like 'Anglo Saxon' and 'Celt' refer to time periods and language, they are not so much about an actual separate ethnic people. If someone was buried in a 'Celt' or 'Anglo- Saxon' type grave, it does not necessarily follow that they were of that, supposed, 'ethnic' group.