im homeschooled too and i did mine at a school nearby. basically just email the exams officers(you should be able to find their email on the schools website) at all the schools around(including public) and ask if they take external candidates. youll have to do this for the writing, listening, and speaking parts as well anyway. for the speaking youll have to get an adult who is fluent in arabic(modern standard arabic/fusha) and familiar with the gcse syllabus to conduct the exam for you, and they cant be related. my teacher did it for me!