When is she due to graduate high school? US HS diploma is about equivalent to GCSEs so she might be better in Y10.
If she is taking AP classes then she should be OK to go into VI form.
In the UK you can go to college (no uni) at age 14. Colleges do some limited GCSEs but more vocational courses so she could do a BTEC Level 2 along side a couple of GCSEs (most colleges do English, maths and science).
She can then
a) go to VI form
b) stay at college for a BTEC National or A Levels.
A BTEC National will get her to uni but not to the best uni or for something highly academic like medicine.
Where are you moving to? There is a US high school in London if you are within commuting distance.
She might also want to think about uni (or college in the US) if she goes into Y10 and takes GCSEs in Y11 she can go to uni in the states aged 18.
If she is wanting to go to uni in the UK she needs to be 'ordinarily resident' for three years to be eligible for funding.
Depending on finances you have a couple of options such as 1 year GCSE in a private school, or possibly a tutor for a year.
She could also be home schooled and take GCSEs by distance learning.
ICS is well known but I have no personal knowledge
www.icslearn.co.uk/qualifications/gcses.aspx
As legally your dd will have to be in full time education you might get some help with distance learning costs.
One final thing. What is her current high school able to do to help?
Could she study for 1 or 2 GCSEs? I think the nearest place to take the exams would be Mexico.
Can she get her diploma before you leave?