Ok. So is a french person more British than a 5th generation black person?

Surely 'ethnic americans' are Native North Americans?

This is the thing. Ethnicity gets wrapped up in a value judgement. We don't want to talk about ethnic Americans, but are willing to talk about ethnic Britons. Which then implies some rights those ethnic groups should have. But I would dare say the majority of 'ethnic Britons' wouldn't identify as British.

So I'd rather not use it in a way that conflates nationhood with ethnicity.