Just reading through your posts, and I missed these: Since everyone else is happy to come along, we won’t be changing the idea.
Do you really know 'everyone else' is happy to come along? Or is it more likely that they are being polite and don't feel they can say no? Just look at the fuss you've/your chief bridesmaid have made over this cousin not being able to afford to attend? All other guests know that that's the response they'll get if they say they can't afford it, so are in good British style sucking it up but are probably seething about the cost and probably worrying about it. They just won't tell you. Because they've seen how you've responded to the cousin.
She didn’t even ask about help to pay
Why would she want to humiliate herself by asking others to pay for her? Maybe she is embarrassed, maybe she has pride. I could never ask anyone to pay for me. I would just say I couldn't go.
One thing to add as well, she was supposed to be a hen do 5 years ago which didn’t happen due to a break up and we were asked to pay £170 each, without travel and not all inclusive so I think she’s forgetting that here would’ve been a similar cost over all.
Irrelevant. Perhaps she was in a better financial position 5 years ago, or perhaps she has now found her voice and realises how extravagant, exorbitant, selfish and batshit crazy paying that amount for a Hen's Night is. She is older, wiser, and has the strength to call it out.
*Just to confirm, I and nobody is forcing anybody to do this hen do.
This is for anybody who would like to join.*
The problem is, as bridesmaid, she would feel specific pressure to attend the batshit crazy one, as she is (was) part of the bridal party. She isn't just a mere guest. She probably feels humiliated and embarrassed that she can't afford/can't justify that cost and will look like a cheapskate saying she will only attend the cheap one.
Embarrassment and humiliation can result in anger.
it's how she has made people feel
Wrong, your chief bridesmaid who clearly has a lot of money (more money than sense) is the one who has caused the bad feelings by planning this waste of money batshittery. You are blaming the wrong person. If you had just directed chief bridesmaid to have a normal Hen's Night that was free or minimal cost, instead of you being a spoilt bridezilla, this wouldn't have happened. You should have reined your chief bm in when you heard about the batshittery she had planned, instead of having a silly second cheaper do. The sooner we get back to having normal Hen's Nights the better. From experience, the more elaborate and batshit the Hen's Night, the shorter the marriage. Just saying.