After reading all your updates OP, I can see why you are concerned. Your daughter may be 18, and legally an adult, but she's not in her behaviour or emotions. I'd agree with others she possibly has ADHD/BPD, and this makes her vulnerable. She's irresponsible, incapable of managing her own money/passport/budgeting and is likely to put herself into unsafe situations.
No, she clearly can't afford to go on the holiday. She's used savings meant for her car insurance to pay for a holiday, and has insufficient funds to take with her for spending money. £250 isn't going to get her very far.
She's also hidden all of this from you, until the last minute, because she knew really, that she can't afford to go and you'd be cross with her. How she chose to handle this aspect of the situation in itself, shows how immature she is.
Realistically, how can you stop her from going? She's 18, she can pack a suitcase and go regardless if you agree or not. In this situation, I would ensure she has more spending money and buy decent travel insurance for her. Make sure she has enough credit on her phone.
Unfortunately, your daughter doesn't see what you see. Her friends are likely going to realise very quickly that your daughter is a liability, because she's going to need someone behind her all the time. That's going to annoy them, no end. Having an odd sleep over when younger or going out in the evening with your daughter, is very different to being in her company 24/7 for a week. The differences between her and her friends will become apparent to them.