I haven't read the entire thread, but surely someone has pointed out that "British" once meant (and therefore, by the specious reasoning in question, ought still to mean) "Welsh." The Welsh (properly called "Cymry") once inhabited all of modern England, Wales, and lowland Scotland. They were driven out or at least subdued by the "Anglo-Saxons," which is also presumably insulting because it names not a real tribe but an academically convenient grouping of not two but three or four related tribes, who presumably were not terribly fond of each other unless they were jointly beating up the Cymry. Included in "Anglo-Saxons" are also the Jutes and Frisians who, to a certain way of thinking, are made "invisible" by the name "Anglo-Saxon," which they never used

The reason you don't hear Jute and Frisian protests is that the descendants of all four tribes informally settled on "English" long ago.

And while we're at it, we really must get rid of "Welsh." First of all, it can now mean "to refuse to pay a debt," which is a stinging insult to all honest Welsh persons. Equally bad, "Welsh" was originally applied by the conquering "Anglo-Saxons" to the native "British" (also misleading, but let's stay focused) because the "Anglo-Saxons" had adopted the word (after twisting the pronunciation) from the arrogant Romans (few of whom actually lived in Rome), who had applied its etymological ancestor to the Celtic tribe Volcae (as the "Romans" called them in Latinized and thus disrespectful form). The "Anglo-Saxons" applied "Welsh" to all Celts.

Which should be spelled with a "K," not a "C." And as for the Picts, nobody knows what they called themselves. "Picti" seems to be a Latin word meaning simply "painted or tattooed barbarians," implying that, north of the Firth, all "barbarians" (a Greek insult for "people who have no real language") were just a big undifferentiated mob whose woad and tattoos made them all look alike.

"Scot" may have meant "pirate," so it's best to avoid that word. The historically neutral word for the "Scots" that is still in use would be "Hibernians," which unfortunately now means "Irish."

Anyone who doubts that these names and words evolved and developed as stated can start with Wikipedia and delve deeper ad lib.

(Mercifully, "Cornish" seems never to have been insulting, though "Cornwall" apparently meant "Land of Cornish Foreigners." "Manx" also seems OK, except that it's not fully a Manx word; probably it's Old Norse.)