I'm black, speak in a middle classed accent, many people have commented I don't sound 'black' so I get the 'where are you from", the subtext being, you sound English, but you can't be one of us, as you're black.
If you can't see asking someone for their heritage and not accepting they are British is rude, you need educating. Would you ask a white person who spoke in an English accent that question? No.
Let me tell you a story, At unviersity , a blonde, blue eyed girl from Kenya, family had been in Kenya for generations, I rudely as a drunk teenager once said she's not a real African- believe me I will never forget how upset this girl was , tears, screaming everything.
White Australians don't respond to that question, 'I'm Australian but my grandparents come from Liverpool'
White South Africans don't respond to that questions 'I'm South African but my grandparents come from Netherlands'
But non-white people can never really be proper British can we? We have to give a speech with I'm British BUT....
I know many black people because of this non-acceptance, DONT regard themselves as British, despite being British, having parents who were British, grandparents who were British ( pre -independence ) , Great great grandparents who were British.
I'm not native British, I'm black, my ancestors lived in the West Indies under British colonial rule since the 1700's, but I don't feel British to be honest. If I was at that event, I would have answered exactly as you suggest, not because its correct, but because I can't be fucking bothered trying to change people's mind who always see me as Johnny Foreigner and alien to this country.