Hi OP
I deregistered our daughter in Y9 as well, actually she was in school refusal so pretty much deregistered herself.
She started at Cambridge Online in Sep 18 in Y10. We did loads of research before choosing them, they were the best fit for her but every child is different.
Positives; tiny classes, superb teaching, unlimited individual contact between teachers and pupils if needed, all work marked and returned immediately (and copied to us), additional work provided to us to help her where needed, group homework sessions in the afternoons which is really successful and fun, lots of online contact between pupils if they want it.
Negatives; technical problems in Sep18 meant logging on each day was a problem, this was resolved by them going back to their old learning platform, I think, but a new platform is going live in March so fingers crossed. The technical problems weren't the end of the world but the school's response was a bit hit and miss. Once they sorted it we've had no more problems.
Positive/negative, depending on your daughter; the academic standard is high. In subjects where she's excelled this has been brilliant for her, in subjects where she struggles we've done a lot of helping/coaching for her to keep up with the class and not worry about falling behind.
They teach IGCSE so if she decided she wanted to go back into mainstream education before taking the exams there would be a mismatch between what she has/hasn't covered.
IGCSE maths and physics appear to be at a higher level than standard GCSES but that is only a personal perception, don't know if it's true.
Online schooling in general;
Can be V isolating, if you live rurally be prepared to spend a lot of time transporting her so she can maintain friendships she would otherwise have at school, also to sports clubs etc as no PE.
You may end up spending a lot of time with her as she won't be interacting with people in real life anywhere near as much, you might become her main human contact. That can be negative for both of you and mess with the parent/child relationship, also sometimes you just need to get away from each other !
Be prepared for higher bills, having someone at home all day means lighting, heating even if only one room and an extra meal a day, I was staggered by how much food we get through.
If it works it is great, and for us it was online or no schooling because I refused to teach her myself. But do beware of the 2 year commitment due to the different curriculum, I'm not suggesting you couldn't switch back but I suspect it could really dent results.
Sorry for the length of my post! But I hope it helps.