Confusing answer but I’ll try my best: The original inhabitants of Great Britain & Ireland were several Celtic/Gaelic tribes. A thousand+ years ago, a couple of northern Germania tribes- the Angles & the Saxons- landed on Great Britain & settled mainly in England creating a fusion culture with the Celtics tribes (Anglo-Celtic). Scotland, Wales, Northern Ireland & the Republic of Ireland saw far less foreign settlement & retained much stronger native, Celtics cultures. Throughout the former British empire- The United States, Canada, Australia, New Zealand, the majority of our white, European populations are made up of British & Irish lineage & most of those immigrants were from England specifically. The most prestigious & dominant culture in England were the Anglo-Saxons. The rest of the world, therefore, came to refer to nations in the Anglosphere (U.K.,U.S., Can.,Aus.,N.Z.,Ire.) as Anglo-Saxon because of our shared culture, lineage, customs & language. I think here in the USA if you added up all those of majority English, Scotch, Welsh & Irish heritage & those whoseeking families have been here so long they just identify as American (90% are of Brit heritage), it makes up about 2/3rd of all whites. That works out to be half the nation as a whole. I think people mistakenly refer to our general culture as Anglo Saxon. It’s more Anglo Celtic ;)
As I understand it, the Celts were themselves a Germanic tribe who preceded the Angles and Saxons onto the isles.
According to some, the “original” inhabitants (e.g., the Druid stock) are unknown.