English is an ethnicity, that has never (before now) been debated. It is not based on where you’re born or anything cultural. It’s based on ancestry. I’m half English, half Irish, but 100% anthropomorphic badger.
Traditionally, to be British has meant being ethnically English/Welsh/Scottish, like a national ethnicity umbrella, but nowadays it’s used interchangeably with “British citizen” by most, understandably IMO.
To be unambiguously, completely African and call yourself “English” is akin to ethnocide. If anyone can be English, then it’s a meaningless distinction.
To say someone’s culturally English makes perfect sense, but to say someone IS English requires them to be of English ancestry.
If an Englishman moved to an all-English commune in rural Japan and they all interbred for hundreds of years, the (highly mutated) population of that commune would still be English.
If, instead, my wife and I moved to Japan and our children went on to breed with the natives, and their children bred with the natives (and so on and so forth), at some point that English ancestry would be so diluted as to not be worth considering. Pedants will argue that the point at which English becomes Japanese in this scenario is too woolly; therefore, it’s impossible to classify someone as ethnically Japanese. This is the Sorites paradox. Just because the point at which A becomes B is ambiguous does not overwrite the observable reality that B exists.
To close (sorry, I rant sometimes), I’d ask why these people feel it’s so important to be considered English? Does it matter so much to you that you’re willing to erase the legitimate identity of others just to satisfy your desire to be a Tranglosaxon? Do you feel that a version of you that’s English is worth more than the version that’s Indian or African?
That sounds a bit racist to me.