Getting your child's surname changed in the register/on their birth certificate
'as a courtesy' they will change a child's surname from the mother's to the fathers upon marriage, or re-registration, but there are no circumstances where it can be changed from the father's to mother's (well, no, if the couple got married, and the father changed their name to that of the mother, and then they went to re-register the child as a child of the marriage, I think you might manage it)