Some children with ASD are in special schools, some in autism units/classes attached to mainstream and some within mainstream itself with support.
The setup here is patchy and you can be very lucky or unlucky depending on what's available locally and depending on the supports your child needs. There is a shortage of available places in special schools and units - there are children with no school place for Sept. This has all been in the news a lot lately with the government trying to fast track more places - so you could have a look at news stories over the last few months for more info. But in some areas this won't be such an issue.
For autistic children in mainstream classes it's also difficult, especially if the child needs lots of additional support. Where we are, access to SNA support in mainstream is generally shared between a number of children, and teachers and students are thus under a lot of pressure. It's hard for an SNA to provide adequate support to 3 or 4 children all located in different classrooms for example. It actually translates into no additional support for much of the day...which may or not be okay depending on the needs of the children, but in practice is often very far from okay.
The model the government is promoting is inclusion, but they are failing to provide adequate supports to allow this to happen properly!
Teachers are begging for more SNA support (in particular) for their mainstream schools and they're just not getting it. Spending ages filling in application forms only to be rejected. It's a disgrace tbh and I feel very sorry for the children involved. Many are being very let down, not by the teachers, but by the government.
I would echo the advice of a pp and visit the school you are interested in attending. For all the usual reasons, but also because - if a unit is the setting you choose - some of them attached to mainstream are geared towards autistic children with an intellectual disability and others are more suited to those without, and you will need to get a good match for your child.