Mainstream is way to go, for now
I am delighted that you want to work in the special needs field; not enough teachers consider this. However, even with your background in a support role in a school for children on the autistic spectrum, I would suggest that you do your NQT year in a mainstream school, for two reasons. First, it is good to work with a cross-section of the pupil population for a year to provide a reference point for your future work. Second, if you find yourself drained after a few years working with autistic children, even though it's rewarding and you are doing a worthwhile job, you may feel like a period back in a mainstream school, perhaps to gain some management experience. This will be much easier if you have undertaken your NQT year in a mainstream school.