Sportie we could talk about sex discrimination, depending on when the promotion incident was, we could talk about constructive dismissal based on your lack of pay rise and you being expected to take on supervisory and other responsibilities while on the same wage as others, and if you complain and they sack you we could talk about unfair dismissal.
So you have a lot on them. However, if you can't afford to leave/bring any kind of case against them, what you are entitled to becomes less important and what is realistic and achieveable is more important.
It sounds as though they do really need you, so you could risk it and go to them, request an increase in line with your responsibility levels and threaten to leave if you don't get it. They might call your bluff though, so you need to be prepared for that.
You could refuse to carry on working under those conditions, register your reluctance and threaten to bring a constructive dismissal case. But they may not care, they may take the chance that you won't pursue it to the end as it is costly financially and emotionally, and they may string it out.
Or they may sack you for complaining and again take the chance that you won't go through with a claim.
I think you need to have a think about whether might be prepared/able to bring any kind of claim/how effective you think a legally worded letter would be, and also whether you can risk being out of a job.
It sounds as though they do need you so the idea of losing you might be worse for them, at least in the immediate future, than a claim being brought, so threatening to leave/not to perform any duties over and above others on the same wage might work.
It's risky though, so difficult to advise if you really think your employer is not remotely bothered by the prospect of legal action and you are not in a position to bring any anyway.
I am aware that's probably not the definitive 'do this' advice you are after.
Have a think about the things I have mentioned as possible options, what do you reckon about the success rate of any of them?