I left in 2019, when it became clear to me that H-UK were taking a strongly pro-genderist stance which IMO is incompatible with the principles of secular humanism (particularly in the pseudo-spiritual and pseudo-scientific aspects). Even if you disagree, and think it is compatible, it's certainly not something that by necessity follows from secular humanist principles. Taking this position means they can't represent the views of all secular humanists on this issue, or even of all H-UK members. It would be like declaring that humanism means being vegan, and non-vegans aren't welcome.
I used to be a member of the national Humanists UK, and also an active member of my local group. I went to the local talks and social events, made friends there, and got peripherally involved in local campaigning on the RE curriculum. I even attended the charity's training course and became an accredited Humanists UK school speaker, and went into schools when they requested a Humanists UK speaker to give presentations to their students on what humanism is, as an example of a non-religious way of life/philosophical conviction.
I don't give myself for free to Humanists UK any more. Why would I give my time to an organisation fighting against my interests?
(The rest of this post is slightly tangential, and about my feelings on Humanists UK's approach to controversial issues more broadly, what it seems to feel its purpose is, and my opinion on that — might want to skip if that doesn't interest you.)
To be fair, it also makes me slightly uncomfortable that Humanists UK doesn't attempt to represent the "broad church" that is humanism when it comes to other controversial issues, even though it's meant to be the national charity for those with a humanist outlook.
For example, it takes an explicitly and unequivocally pro-choice organisational position on abortion, which, even though I personally am very strongly pro-choice too, makes me feel they're representing something more/other than simply secular humanism. Being pro-choice re: abortion does not necessarily and intrinsically follow from humanist principles, though for me it does.
I think it's possible to be a secular humanist, to believe in improving society for everyone by making public policy through rational and compassionate scientific and philosophical argument, to share all of the basic tenets of humanism — and yet be against abortion on demand, for entirely non-religious reasons. I very much disagree with those views, but I think that while Humanists UK should explain the (probably) majority pro-choice view of their members and of humanists in general (or those who think like humanists, but may not describe themselves as such), they should also represent the breadth of opinion among humanists on matters like this, which would include the opinions of people who've used the same humanist principles and come to different conclusions.
Non-religious people who take positions which argue against free availability of abortion are poorly represented in the media as it is. Whenever the media need someone from that side of things, the easy option is to grab a religious pro-lifer. This results in the general impression that the only antis are religious or conservative people — much like how (especially in the US, it seems) it's easy for people to assume the only reason anyone would question extremist genderism is that they're right-wing, extremely religious or both. There aren't many humanists who are as extreme as religious pro-lifers, but there are quite a few who express concerns.
I don't have a problem with Humanists UK accurately representing the majority view of those within the organisation or of humanists in general, and the arguments made for that position, but recognising the pluralism within humanism and the fact that different humanists can come to different conclusions based on the same principles and evidence is important IMO. It's kind of the whole point of the damn thing — to think independently, rationally and compassionately about issues, without having to follow an institutional line about what we're supposed to believe.
Humanists UK doesn't currently represent humanists — it represents a subset of humanists with a particular set of views. There aren't a whole lot of different humanist organisations to choose from so you can find one that fits your worldview, like joining a church, and I wouldn't want there to be. A humanist organisation should centre humanist values and principles, not dictate specific conclusions that humanists must agree upon — the starting-point and the navigation methods, not the destination.