OP I would also consider a private school - or maybe even sitting exams as a privately entered candidate.
If she has been studying for iGCSEs, you need to know (you may already know of course) that the majority of English state schools do not do these; and the syllabi are different.
My subject is MFL, which you would think was fairly universal, but if a student had worked towards iGCSE and then tried to sit AQA GCSE, it might be unnecessarily challenging. AQA GCSE has translation from and to target language; edexcel iGCSE does not. AQA has some questions in English; iGCSE has them all in TL; iGCSE has a grammar task that is not in GCSE. GCSE has a much longer essay question with less support (only two bullet points). GCSE is tiered so the F tier is relatively easy but the H tier is challenging; iGCSE is not tiered, so the questions range from easier to harder and IMO it is overall easier for a really able student.
What I am trying to say is that most schools will have been teaching translation from year 7, but your DD probably won't have done that as it's not on the exam she is working to.
This will be multiplied by many times in other subjects - different topics in history, different texts in English, different aspects covered or expected in science and maths.
If I were you I would make every effort to keep her where she is tbh; failing that, I woudl try to find a school that will take her in year 10; failing that (would probably need to be private school) I would enter her privately for the iGCSEs she has studied towards, paying for some tutoring if needed. She could still go to the local school I guess.
Sorry for long post but I fear some people (not saying you OP!) don't understand the skills and knowledge of exam technique needed (and think that "speaking French" is enough to get them a high pass at GCSE or even A level.