In one presentation, given by a member of the LGBT network at the Public and Commercial Services Union, which represents civil servants, attendees were told: “There is no legal or even scientific definition of ‘biological sex’.”
This is paradigmatic of TA misuse of language. In a literalistic sense it's correct to say that the term 'biological sex' is not used in biology. That's because the term 'sex' has a technical, precise definition. Outside of biology the term 'biological sex' is used to refer to the word 'sex' as used in biology, i.e. with its technical definition within the broad fields of biology.
So, the claim that there's no legal definition of 'biological sex' rests on assertion that the term 'biological sex' is not itself used in the field of biology, namely that there's no biological 'biological sex'. This is obviously semantic game-playing. It's so silly and transparently dishonest that it's impossible to pin down TA's into making clear that this is their argument.