I’m really confused about your Dds education. If she’s currently studying for GCSEs then she’s the equivalent of 4th year is a Scottish High school. So she can sit her GCSEs May 2018 and then start 5th year in high school in August. In some schools she can start 5th in early June.
She would sit CFE Highers in 5th year in 2019 and more highers or advanced highers in 2020.
So she wouldn’t be going to University until September 2020, three years from now.
Given that she has been so unwell with such a serious illness, I think it’s too early to worry about what’s she might be cooking three years from now. I think that getting her well is more important that teaching independence.
Kids move from the English to the Scottish school system( and vice versa) all the time and cope fine. Children coming up from England are usually older than the others in their year but it’s no big deal.
There is also far more flexibility in placing children in the year that meets their educational needs, rather than by age as in England. Remember that many children have deferred entry so are often one full year older than others in their year group.
In summary, I think the educational issues are relatively straight forward and the school will be able to handle it and her transition.
Her health issues OTOH are a big deal, much more serious and potentially life changing if not life threatening .
I think you need to prioritise her health - keep her at home and send her to a good day school. There are many to choose from.
Boarding school is great for the right child. Nothing you have said makes me think that it’s right for your DD.