You might struggle to get a diagnosis for autism, but that doesn't mean it's impossible.
Part of the investigation often needs a history from someone who can list your struggles and had known about them in detail from childhood. This can be a parent or a sibling or a close family member but they do need to be able to explain how your struggles have put you at a significant disadvantage all of your life and tie it back to autism.
They also need a family history to rule out other possible conditions that display similarly like early infancy infections, birth complications from your mum, foetal alcohol syndrome etc.
Now I know not all providers require so much information but this was my experience through the NHS for my diagnosis and to my understanding is the standard for evidence they should be looking for before diagnosing you with a lifelong developmental condition, which is why I'm saying that for you personally it might be hard to get a diagnosis but not impossible as it depends who you have available to give evidence at the early life history appointment, and depending on your own trusts processes or whether you decide to go privately.
I agree with other PPs that the reason that you were probably referred to a SEN school is what we would now call dyslexia but back in the 80s they did not so much care about the distinction of your diagnosis but rather the presentation of your need. They also probably wouldn't have recognised if you were autistic or not as autism was previously known as infantile schizophrenia, which evolved into autism for high support needs individuals and aspergers for low support needs individuals, but we now have a wider understanding that it is all just autism and different people fall within a different spectrum of need within if they meet the criteria for diagnosis.
I dont think you're going to get an answer about this from looking at why you went to a SEN school. You had a legitimate need to be in one, but you still have legitimate needs and differences from the general population and if you want to know what they are you need to speak to your GP and ask for a referral.