From what I understand individuals are not subject to equality law. Only organisations. And, again, from what I understand mumsnet does not constitute the type of organisation they are talking about.
So misgendering (by individuals) could only be considered a hate incident (not a crime because that has to involve an actual crime) if it was done with deliberate malice or in a prejudicial way.
And, again, I don't believe it's actually illegal. it's more for statistical purposes.
Saying Lily is a man is completely correct because they don't have a GRC. Saying they are a biological man, would still be correct because they are. Even though, to all intents and purposes, they would be treated as a woman.
Otherwise doctors and surgeons would be getting arrested, left right and centre for correctly identifying biology as male or female.