The new-style GCSEs open up more options to home educated kids. It used to be that with just a few exceptions (e.g. maths), GCSEs always required coursework, which had to be validated. That was difficult and expensive for HE families to arrange. As a result, IGCSEs (which were and are exam-based) were the usual choice. This wasn't a huge problem, as IGCSEs are universally accepted as equivalent to GCSEs - in fact, many private schools do IGCSEs.
But now that GCSEs are moving away from controlled assessments, in more subjects you will be able to choose between GCSEs or IGCSEs. You can choose between several exam boards according to which syllabus you prefer. However, bear in mind that it can be tricky to find exam centres which accept private candidates. The availability of an exam centre may dictate your choice of exam board.
There is a wealth of information here:
he-exams.wikia.com/wiki/HE_Exams_Wiki
Once you've had a read of that, join the national Facebook or Yahoo group to discuss details. It's also sensible to be in touch with other local home educators, who will be able to tell you what's available in your area. Private schools, independent exam centres, colleges, and sometimes state schools are able to accept private candidates; however, their willingness to do so tends to change from one year to the next so you'll need to check back again when your daughter is closer to sitting the exams.
In addition to, or instead of, tutoring your daughter yourself, you might choose to outsource some subjects. Some colleges offer part-time courses suitable for home educated kids, which would be free. In some areas, a tutor (often a home ed parent who has specialist subject knowledge) may offer a tutor-led study group to help kids prepare for exams. Or there's distance learning.