Sitting GCSE as an independent student without an EHCP

My 16 year old daughter has recently been diagnosed with autism and ADHD. She has not been in school for 2 years now. Up until then she masked her way through life and we or her schools had no idea she was autistic. She wants to sit her maths and English GCSE exams but is not affiliated to any schools. Any idea how to do this please?  Thanks