If said British Guy has a kid, he's not raising the kid to speak Cantonese and he's gonna be sending it to some special "international school" segregated from the locals. Mainlanders in Hong Kong are Chinese people living in China. Anyone who calls them an immigrant is a liberal white supremacist. On the flip side some Russian family that's been in Heilongjiang for 3 generations isn't going to be called expat or immigrant, they're ethnic minority Chinese living in their own country.

It's a legacy of colonialism. Europeans came to China as colonists, and just as they did elsewhere, did not and will not consider assimilating. An expat is simply a colonist who has failed in his attempt to eradicate the local culture. I will bet you actual money your hypothetical British guy speaks poor Chinese (if at all) and spends his spare time posting about how he wants to genocide our language and eliminate our writing system. I've met so many of these people. A great many of them are open white supremacists. I won't call these people immigrants because they aren't immigrants, and the word expat should remain as a term of contempt for these people that should not transfer to actual immigrants.