You must have missed the part of that document which said, very clearly (my bolding/highlighting):
The UK government defines gender as:
a social construction relating to behaviours and attributes based on labels of masculinity and femininity; gender identity is a personal, internal perception of oneself and so the gender category someone identifies with may not match the sex they were assigned at birth
where an individual may see themselves as a man, a woman, as having no gender, or as having a non-binary gender – where people identify as somewhere on a spectrum between man and woman
And I'd say that, erm, most people would find it extremely easy to derive a definition of "woman" (in this specific context) from the above excerpt. But in case you cannot, I've done so for you already, a few days ago (do a search).