I'm fascinated to read how others are interpreting this question. If I touched a nerve with the term "overthinking" it was not intentional.

To me, the census is asking about
my ethnicity, not my grandparents. I was born in the US, as were my parents and my grandparents. I have great grandparents from Hungary, and great great grandparents from England, Scotland, Germany and maybe somewhere else that I don't know about. To my mind, none of that makes me English, Hungarian etc. On every other form in the UK, I put white, other, so that is what I did on this form. I think the people at the census dept are wise enough to understand that a white american is most likely of mixed white origin. We are the melting pot after all!
This question seems to touch people differently as it's tied into a sense of identity. Put whatever makes you comfortable, but I doubt the British gov't expects anyone to lose any sleep over it.
