Heads up for others who have donor conceived children. Just found out that the census is asking for country of birth details of the biological other half of all children (not just the details of where the legal parents were born).
So if you have used donor eggs or sperm, by law you are required to provide information about the donor.
" This information is collected from everybody, even children whose parents have already provided their country of birth. This avoids making assumptions about family relationships."