You could see if there are any home school support groups in your area, they will at least know which schools are friendly to external candidates and may even have their own centre set up. I sat A level Spanish this year at such a place:
In my case I was studying the course online through NEC so my exams were booked though them.They guarantee that they can get you a place in one of their partner centres (though your nearest one may be a long way away):
It's more expensive than just self-studying and booking the exam yourself though of course.