OP, some good advice on here already. A few things I'd point out:
all: “avoid discriminatory or exclusionary language and behaviour at all times”
Discriminatory language and behavior has been unlawful for some time, although I imagine "behavior" would be more likely to be determined by internal policies. The employer taking action against "behavior" would probably have to prove that it was discriminatory.
As far as I am aware, IANAL, "exclusionary language" is not a legal term. So, perhaps madey-uppy to proscribe hurty words?
"arrange informal/formal interventions where required, including facilitated discussions, mediation, counselling or changes to duties, location or reporting lines. Use the misconduct procedure if you have serious concerns.”
This is all normal management and disciplinary stuff. Misconduct would normally be escalated to HR, if I'm not mistaken. As a senior manager, I would expect you to be able to handle situations like these.
and so a female colleague telling a trans-woman they cannot use the female toilets would presumably not be exclusionary, as it has been proven in law.
Why would a female colleague be telling another colleague which toilet to use? This should be in your organization's policies, following the law. If a colleague was not using the correct toilet, I would mention it to a manager (which is you). Then you can get HR involved and organization's legal dept. if necessary.
Which then begs the question of how (and indeed why) they can extend the same rights to gender identity/expression, which are not protected by law
I don't understand this question. The same rights as what?
we have female, male, gender neutral and disabled toilets available in the office,
Then, your office is covered, and in line with the law.
As @Wishesandhorses says: I suspect this is the next line of attack against women - to try and claim that women voicing their rights and protesting the breaking of their legal protections is in itself an act of harassment - it seems to be a common agreed theme appearing.
Yes, many organizations will attempt to get around the law this way.
But, I'm not really sure what problems you are anticipating, or what you feel you might have to "voice" such that it becomes a problem. The only way anything could potentially manifest as a problem would be around pronouns. Are you worried about pronoun usage?