Op, is there any distance learning she could do from USA so she would finish with the high school diploma?
A few bits you need to fully understand about uk education that I haven't seen mentioned (but I've only skim read so apologies if it's already been said).
GCSEs or equivalent are needed so do not underestimate the importance of the year 11 in school. All college, uni and professional jobs immediately look for gcse in Maths and English plus some will like a spread of other subjects - it's used to be minimum of grade C (a* being best) and it's now changed to number grades, higher the better but minimum 6 (I think, it keeps changing).
To go to college or sixth form to do alevels or a vocational course she can enter at different levels - from what you say she would suit level 3 voc or alevels and she would need min 5gcses grade c/5or6 to meet the entry requirements.
She could do a level 2 or even 1 at college first but she would be bored crazy as they are too easy for her. (I've had students have to do this before in her situation and it's so frustrating for them and us). If I'm honest, it also then doesn't look like they are naturally gifted if they've done a level 1 and level 2 and 3 at college, it looks like they've struggled and worked at it - this could potentially impact a future career.
The gcse course in schools is a y10 and 11 (some even start in y9) as you need the time for the content and courseworks before sitting an exam in it all at the end of y11, most kids will do 9 gcse subjects. Whilst she is in school in America, her subject content will be different - you need to be following the specific course specifications. In USA that specification is dictated by state so everyone in that state does the same content, here each school chooses the awarding board they wish to use, as a result even moving between local schools is an issue as they may have been learning a totally different Syllabus.
Honestly, moving her to a British school for that y11 could be detrimental in the long term - not having the GCSEs in maths and English would mean she also can't then transfer her quals back to the equivalent of a us school diploma either. I can't remember the exact info but I had my quals transferred to Canada and I think my 9gcses were the equivalent of a high school diploma and then my alevels contributed to the university equivalents (happy to be corrected with this as I don't have the info to hand but it was def significant).
I would be seriously looking at any online schools in the USA she can follow in the U.K. or looking at a homeschooling option.
I would also look at just doing the English and maths core as GCSEs here (evening classes /day classes in college etc) to get them signed off (but rules are must be in education until 18 here so might have issues with college agreeing to this).
Then get the high school dip signed off and transferred over as a equivalent qualification for uni application purposes.