Year 10 is for fifteen year olds. So she will be the oldest in her group but she will have enough time to settle and learn for GCSEs
Year 11 is when you will have your GCSEs so you won't get enough time to prepare ( GCSE years are Y10 and Y11) as you will find it difficult to settle in ; still it is not impossible just really difficult.
You have to choose subjects to study for A Levels (you also have options in GCSE but it is not very important). The subjects you choose should depend on the course you want to do at university. So, if she is planning to start schooling at Year 12 then she must get really good grades ( >75%) in Maths , English and the subjects she wants to do at A Levels.
If she does her GCSEs then she must pass in Maths and English language and get good grades in the subjects she wants to do at A Levels.
Starting at Year 12 is better because for A Levels you learn for 2 years ( Year 12 and 13) . So she is on the same level as her peers. But if you are looking to go to university in UK, then you should have stayed here for 3 years to be considered a home student (tuition fee - £9535 per year for all courses in all universities) or else you will be considered an international student ( tuition fees is really expensive and is different for different courses and universities).
Since she would have only been here for 2 years, she will be considered an international student.
She can however start uni at 19 years old as a home student. In that gap year she can work/prepare for uni applications etc.
Remember A Levels can be hectic so she should choose what's best for her - if joining at Y10 will give her more time to settle then do that ! Or if she can manage the A Level workload along with settling in a new country then that's alright.
All the best for her🙂!