You mentioned that your daughter's self confidence is fragile and you worry it will crumble if she struggles at school. Are you in a position to be able to home educate her? If so, perhaps it would make sense to start off with that first while she settles into living in a new country. She can have all the extra help she needs, and the work is guaranteed to be at the right level for her because you would be choosing it and adapting as needed. Home ed is increasingly popular here, especially for kids with special needs who aren't able to access a suitable education at school.
Your daughter could get out and explore her new country with its wealth of historical sites, and make friends with a few kids at a time rather than dropping into the intense and sometimes relentless high school social environment with all the added pressures that brings. Assuming she wants to go to school eventually, that buys you time to look around at schools and find one that suits her - if in England, she doesn't have to attend the nearest school, though some schools will be full, which restricts choice. Having got the lie of the land, if needed you could move to another area with a school you like better.
One point to bear in mind for the longer term is that preparing for standard exams becomes all-encompassing later in high school. The way this is done at school is very rigid, so it is inadvisable to change schools, or move from home ed to school, during this time. If in England, you would want to ensure your daughter was settled into school by the beginning of Year 10, which in her case would be shortly before her 15th birthday. Home educated kids can also sit these exams, and it is much more flexible for them as they can do them at any age, choose from a wide range of subjects, and spread them out over a few years to reduce the pressure... BUT organising that is a lot of effort, and it can be expensive to do out of school.