@bendmeoverbackwards the BTEC does sound like a good idea. A consistent group rather than 3 different ones might be easier for her to settle in. The uniform thing is frustrating but so many autistic kids end up coping better than expected with blazers and ties, so it should be one you can break down and overcome. A designated "personal uniform" is a good shout. Maybe start with black leggings and wash her 2 favourite dance tops for the first week, see what everyone else is wearing, and buy more then.
Making a list is a good idea. I would also talk with her about how the first days or week of a school year are always super challenging for most autistic students. An autistic brain usually hates transitions, so the negative things she is feeling about new places may be an automatic reaction her brain is making to the change. A bit like how you'd address anxiety by talking about the thoughts and feelings as separate to the physical and very real sensations, try to decouple the starting college or school from how she would feel once settled. In a panic attack those physical sensations are overwhelming, but they don't actually come from the stressor, they are a separate thing. So her negative reaction, while real and overwhelming, might actually be coming from the newness rather than the setting. It is hard to know which, so how about she sticks with the setting for a couple of weeks just in case it's the former?
Whether A levels at private school are the right fit for her academically is tricky with her attendance. We have steered away from private school for our autistic DC because of a norm we perceived of more social (compliance) and academic demands - more hoops to jump through I suppose, and they all use spoons. It works well for some kids though. I would imagine Dance has its own social challenges with group work. It sounds like her social skills are pretty good.
As she has struggled to attend in the past, I would weigh quite heavily whether the settings have somewhere she can go to escape and get some support, when feeling overwhelmed. That might be a key part of keeping her attendance up. Good luck.