I would read it as meaning the sex I was born with an answer accordingly
Yes, of course.
...and members of the opposite sex to you will read the guidance notes which clarify that you can put down the opposite sex if you have self-identified as such.
And presto, the sex:female category is a useless jumble of both male and female people. People who have provided their actual biological sex, and people who have provided the opposite to their biological sex. All muddled in the same category, indistinguishable one from the other. Utterly corrupted data, impossible to retrieve the facts from the anti-facts.
It's designed to encourage honest people to answer factually and in good faith so that their data can be purposefully conflated irretrievably with the people who are encouraged to answer with the opposite of sex.
You can't conflate facts with fiction unless both parties willingly participate in the conflation, or you compel one party to provide facts, and exempt another party from the same.
The exercise is designed to conflate both sexes under the category sex: female. The conflation either happens with willing and naive participants, or it is forced.
But the objective is conflation of the sexes.
The ONS have rejected the alternative, which is to require the opposite sexes to truthfully differentiate themselves.