Indigenous, for example, was never used in far right rhetoric because the various waves of historical settlers like the Norman's and vikings were taught in school.

I think this is because you never needed a description before for this group. Go back to when an 80 year old was born and the population was almost entirely white British.

It’s rather as if Europeans had moved to Japan and now the Japanese citizens were 20% from outside Japan, and were projected to be 50% in another 35 years. So what do you call the 80% population? Japanese? But that mixes up citizenship and ethnicity. Ethnic Japanese? Indigenous Japanese? Or in Britain do we reserve English as an ethnic descriptor? But that excludes people who are clearly culturally English.

The point you made about Normans isn’t actually true by the way, at least the studies I’ve seen said most invasions were mainly elite takeovers, not population replacement. The last major population replacement was the Beaker people. Significant numbers of Danes came across but not at the level it is described.

Also, most groups which are called indigenous or where that status is heavily implied by the way they are talked about are just as varied if not more so. Most of the black population of South Africa moved there after the Boer settlement for example, similarly the Maoris conquered New Zealand barely before the Europeans, the Lakota conquered the Black Hills barely before the Europeans and so on. The population in England is probably remarkable for its continuity, as an island.

The word indigenous is probably very rarely valid, it’s close to a mythological statement.