'certainly NOT a national staff consultation'
Not sure which part of the charity sector you are in, but surely the primary purpose of the organisation, are its aims.
Does Stonewall's objective have any relevance to the aims of the organisation?
Also it isn't for the Board or HR to say you have to have an agreed set of beliefs to work there.
And law cases have made it clear that both trans and gender critical "beliefs" are equally protected so as the employers should not be putting one in front of the other.
However, suspect that the charity sector is even more captured than any other sector - sadly.
Sounds like you are being very sensible and informing yourself.
Are there any other members of staff you can talk to, and share concerns.
And yes if you are a unionised work place the union should help ... but many of them have been captured.
I would have thought the potential of reputational damage to the charity would be a good angle. So many newspaper reports about groups disaffiliating.