OP you are not unreasonable at all. Mumsnet is having another weird mood.
You should not have to contribute to your ex husbands marriage in any way, shape or form, and that includes paying for plane tickets for your daughter to go to his wedding.
Your ex is her father. Not only should he pay for everything so his 18 year old daughter, who is in school, can come to his wedding it should not even be up for discussion at all. This is ridiculous.
On top of that you can't just invite one person of a couple and expect the partner not to come and for this to be ok. It does not matter if its a 5 year marriage or a 8 month teenage love. It's just fucking rude.
She is also going to be alone at the wedding and as a father surely he would want someone to travel with her and keep her company. She's not a small child but she's not an adult either.
That they won't pay for the boyfriend is more understandable, though honestly as a parent, and considering the boy is in school with her, I would have pitched in so it's reasonable. If you can have a fancy wedding you can have your daughter and her boyfriend there.
Your ex husband is a coward and acting like a shit dad and your daughter is obviously upset by it.