Being told my job description was changed after I joined

Where has OP been selfish?
When you go to UNITE landing page, it says in bold letters Why join? and cites legal protection, benefits which protect you and your family and access to online training.
Anyone unfamiliar with unions will assume that that's what they do. Nowhere does it say "we won't represent you unless you pay your fees for 30 years and show ideological alignment with our cause". So how was OP supposed to know that?
I have also never seen it mentioned on staff posters around my workplace. It is only because I tend to deal with union reps when supporting others with employment issues that I know roughly how they operate. But even I was surprised at the level of aggression towards OP and I can only imagine how she feels having come here for help in what sounds like a tricky situation just to be faced with this attitude.

It is just a money making scheme for blokes and their mates.

I do not believe that someone in her position would not know how a union works, how it gets funded, or why her attitude might be seen as grasping. Failing that she could tap into Google and get the info that way. Or the Union’s own website. She is even entitled about doing a bit of research for herself. One of her responses was aggressive, she is entitled and is coming across as stupid. You explain it to her how unions work then rather than coming to her rescue.

I joined the union years ago and when i needed them they couldn't help at all. Seemed a waste of money and I'm tempted to leave. They seem to.want me to vote for something every other month though.

They aren't reacting to the first post. They are reacting to posts by the OP that show she thinks she should be able to join, get the full benefits and leave straight away, and claiming that unions are there to support everyone (and by implication including her when she doesn't want to pay more than a month or two of fees). Some of her posts have come across as extremely entitled.

They usually won't help for the first 28 days. Or that's the rule for the 4 where I work.

Unison, Unite, GMB and Prospect.

Find your rep and ask. Or look online for the one that is recognised by your employer.

I probably would be upset, but if I'd been obtuse and showed my selfishness so boldly, I'd deserve it.

You dont need a union for this op. Assuming nothing has been confirmed in writing about the pay scale discussion we'd entertain the rep for a while before pointing to however the job role flexibility clause is worded that is likely to be in your contract somewhere and sending them on their way.

First step is you need to explain how your other tasks are being affected, but you do like the idea of progressing just not to the detriment of the wider role snd yourself, and ask for a discussion on how you can both address it.

If you are in teaching, there are non-politicised organisations like a Union that can help you.

Frankly, my experience is that the Unions are not worth the subscription fees. When you need them they are useless.

Because I don't find OP's posts aggressive. She is quite right to ask whether unions only support people who ideologically aligned to them and what is the point of them if they won't help because up until that last reply, not a single person actually took time to explain how unions work and, instead, insulted OP.
I will hazard a guess that if you were asking someone for help and they called you stupid and entitled, you'd be somewhat upset about that.

Horrible reply. Disgusting actually. Totally not needed.

Why are you only criticising replies from others and not the OP's rude and aggressive replies?

@tilyougetenough you are coming across as ignorant and entitled.

I've paid my union dues for over 30 years. You're only interested in being in a union when it suits you. And you're being v unpleasant to people on here who are pointing out how a union works.

I am sure you know everything about every topic on the planet but some people are not informed to the same level. It's not entitled to ask a question about options when one finds themselves in a difficult situation. As I said, what's been made abundantly clear is that unions are not good organisations to be a member of as their members seem to lack any form of compassion. I can only hope that my very niche professional union is different.

I'm a retired public sector worker and a former union rep. Wherever I worked my induction would include the contact details for the unions at the place of work. Employees were encouraged to join, so that they had a the benefits of being in a union. I always joined straight away because you never know do you? And sadly, the OP has discovered the hard way.

@CatzaNot angry, just amused at her cheekiness/lack of intelligence. OP was attacked for entitlement not because they are horrible organisations. Guessing you have done the same as OP wants to.

Yes they are, but the reason they are there is that existing, long term members have paid in to fund training for the union reps (who are largely volunteers on top of their main job) and to fund legal advice etc. They've paid to fund campaigns that improve salaries and working conditions for all staff, including you. Many of the members who have paid for these things are quite likely paid less than you.

What you're planning to do is morally the equivalent of walking up to lowest paid member of staff on your team and demand that they pay from their own pocket for you to get legal advice for your problem (that only affects you), because you think they should pay for it, then throw a tantrum when they say why should they pay to support you when you haven't ever done anything to help or support them.

OP asked if she should join a union. She clearly doesn't understand how unions work and the reasonable course of action is to alert her of how they work. The first comment immediately made it personal by saying "I won't represent you" with zero explanation. Some bizarre comments then proceeded to call her names, insult her intelligence and suggest she is not capable of doing her work. I'd say it is pretty unfriendly and uncalled for.
OP is in a difficult situation asking for information and advice. She is not "entitled" or CF. She simply found herself in a difficult situation, on a brink of burnout and has never been in this situation before and, therefore, is not sure where to turn for help.

I really don't think people are being unfriendly, they're reacting to the cheeky fuckery attitude the OP is displaying.

I think you're being a bit too much of a pedant. Most people understand the gist of the posts making a comparison with car insurance or a break down policy.

The point of a union is for members to mutually support each other.

If you only join when you need support then immediately leave you're taking advantage of people who are members all the time.

Sure, but they'll charge you appropriately for the call out.

They do help.

But most don't for ongoing issues.

That's why it's worth being a member.

So I there is issues you have help.

You seem quite entitled and are very welcome to quit or join whenever you want. But might find when you need help you don't have the help, because you weren't a member before any issues started.

You join a union for the long term in case you need assistance in the future.
You don’t join because you have an existing issue. The union won’t help with that.
It’s like taking out insurance after your house has been burgled and expecting them to pay out.

Well, I think it's been abundantly clear that unions are not healthy organisations to join looking at how members attack OP for no reason whatsoever.
@Irridescence I think you need to have a long hard look at yourself and maybe get some anger management courses. Nowhere did Op say her job is low paid and I am not entirely sure what it is she is going to get "found out and sacked" for...

OP, I would get a copy of your contract and your job description and give ACAS a call. Their helpline is extremely useful and friendly. Depending on whether your "therapeutic" setting is private or public, pay grades could be more or less blurred. Hopefully you'll get more support and a plan of action with ACAS