but non binary isn't 'presenting as the opposite sex'
that's the point.
When the EA was written barely anyone had ever heard of being non-binary. Therefore the definition of the PC of gender reassignment was not written with that in mind. It's possible if taken to tribunal someone could argue that it is thematically similar so the PC should apply, but then that edges into the whole SC ruling - can you backdate and interpret what you think legislation should mean? Or do you have to go with what was intended at the time (and if out of date, campaign for it to be updated). SC ruling found the second.
Besides which the test of discrimination because of a PC is whether someone without the PC would be treated the same.
e.g. access to female toilets.
Trans positive argument would be that the comparator to test the scenario against is a 'cis' female - cis female would be allowed to use the toilet, they aren't solely because they are trans, therefore they are being discriminated against because of their GR.
GC argument would be that the direct comparator should be a man, because trans women are still biologically male. A 'cis' man would not be allowed to use the women's loos, therefore a trans woman not being allowed to use them is being treated exactly the same, and not being discriminated against.
I imagine the same would apply here.
NB person's argument would be that they are being treated differently compared to a cis woman who wants to be called by female pronouns and is called she/her by everyone = discrimination
GC argument would be NB person, however they feel/self-identify is still biologically male, and therefore is referred to as 'he/him', like any other biological male.
Again, hopefully any reasonable person would just agree a compromise. But if it did get to court, would it be considered a breach of the EA? Maybe a few years ago but now, I don't think so, as it would have to contradict a lot of recent rulings.
Currently there isn't any legal protection for being NB in law, and until/unless the EA is updated to include being NB as a PC, it's not appropriate for a HR consultant to insist that something that isn't even mentioned in the EA absolutely is a breach of it. Particularly given the role of HR is to protect the company, not recommend a course of action that could potentially embroil them in legal action up to the highest court in the land to query an untested point of legal interpretation!