@originalShapes,
I think yours is slightly different I am also an expat but I imagine when people address you the conversation goes like this (imagining you live in Asia or Africa or similar)
" Oh so where are you from?"
" British"
" Oh nice, I have been to London once, a long time ago!"
I doubt when you answer " British, or wherever you are 'originally' from, people keep on banging " no but seriously, where though?" I am sure if being white isn't the norm where you live people also ask you where you are from, but I am also positive they take your answer as a fact and don't press it anymore than the initial answer.
If when abroad people asked me,
" Where are you from?" And I said " French" and they were like " Oh okay cool." I would not care at all because it's a normal question.
It bothers me when I say French and people just won't accept that as an answer.
It's like they don't trust that I could be French and I am "obviously" something else. Usually they very obviously already know the mixity and it's just so they can hear themselves say " Oh, I knew it!", I don't get it. Being mixed also come with a bunch of assumptions about my skills and abilities and the things I must like or dislike or know about just because people have pre-conceived ideas about what "people like me" are like, it's tiring, I don't want to have to disclose my background and personal story to strangers, I should be able to go to the hairdresser or the restaurant without having to discuss my skin color and family heritage with everyone and having to listen to people make unwanted comments (as nice as they might be) based on stereotypes and/or clichés they have about people with my mixed background.
I think people who say most people would be proud to talk about their background and not be offended, I am proud of my background but I am born and raised in France I have been to Africa twice as a tourist, feel everything BUT African and so when I say I am French it is me being proud of my background, it is not me wanting to hide "where I am from" or looking to be offended it is just me not wanting to have to talk about something that is almost irrelevant to who I am. It's like saying to someone whose great grand father was Italian that they should be proud of their origin and introduce themselves as Italian as much as British (even though nobody in her family has stepped in Italy since the great grand father immigrated), it's just ridiculous and nobody would expect that. Yet because my "origin" is visible I should apparently be flattered to have people take an interest and ask personal question about my background.
If I went around asking super pale white people where they are "really from" because "surely nobody is that pale around here." I would probably be insulting and causing offense.