Our daughter was a young adult by the time she finally made it to the top of the waiting list. The first contact was with the GP who went through some screening questions with her and then referred her to the NHS autism screening unit. They sent a long questionnaire that required filling in by both her and us. It went through behavioural things as well as why we thought a diagnosis might be helpful. They asked for contacts who could talk about her education and development. That then got graded and they decided that a diagnosis might be appropriate.
It then took nearly four years before she got an appointment. Around a month before it, I was told that they would call me a week prior to her appointment. During the call they asked everything from how the pregnancy went, to if she passed her milestones and how she was at school.
Prior to the appointment they contacted her and explained how the process works and if there was anything about the environment they could change to make her more comfortable.
At the assessment she requested for me to come with her. The first question they asked was what we hoped for during the assessment process. Then they explained that they normally need to see her multiple times and then sometimes they would then need to have a meeting with everyone to decide whether they could see evidence of things that fit the criteria. Only in exceptional circumstances where it was obvious, would a diagnosis be possible on the first visit.
There were two people in the assessment and they both asked questions. They once again went through childhood with her, educational things and how she would respond in hypothetical situations. They explored her mental health and asked about friendships. They explored the type of support she was receiving at university and how it was working.
Our appointment had a couple of breaks for drinks and then at the end when we had gone over time, they said they had a diagnosis and would she like to hear it then or come back next time to hear it. She opted for straight away and was told she was autistic. They gave her some leaflets and answered some questions she'd had.
They then arranged another appointment to discuss it further and start to write the report and a document of specific information and adaptions that university, employers or healthcare settings should make for her. This was a very collaborative process and she was able to add in things later via email (she doesn't do phones) afterwards.
They also enrolled her on the online course they run for newly diagnosed people and other charities that can offer support.
I wouldn't think that a speech and language therapist is qualified to decide if your child is autistic by themselves, even if they discussed it with the ND team. We were told it needed to be observed by two practitioners and they both had to agree with each other. If one has any doubt, it would require to be assessed further.
There was also an emphasis on what we all thought about the diagnosis and how it would affect us. The document to "introduce her" and suggest reasonable adjustments was a very important part of the diagnosis and nothing was hidden from her. They explained what they were saying about her, why they thought it and how she wanted people to treat her in different circumstances. This is entirely lacking in what has happened in your child's case. If nothing else, it's left you feel shocked, unsupported and questioning if the diagnosis is correct. None of this is right, even if your area has given the speech and language services the ability to diagnose autism.
If you were just told that they were going to write up the report for the "diagnosis", it's possible that they were talking about the speech and language elements as the "diagnosis". We had years of speech and language therapy for our oldest and he had a diagnosis of Auditory Processing Disorder. Maybe you have just misunderstood.
It might be different because of your child being under 18 but I would expect some explanation and support for you both about having been diagnosed. I would also expect there to be some formal recommendations for adapting the school environment and ways they relate to your child. I would have thought to prepare it, they would go through their findings and get your input on the changes too.
Be gentle with yourself, it is an emotional time. Even though our daughter had requested the assessment herself and we were quite convinced that they were going to think she was autistic, it took time to process.
Remember that nothing has changed about your child, they are the same as they were before the diagnosis. Now you just have the way to explain to others about them and autism is a protected characteristic in law.
It doesn't mean that all the dreams they have for the future can't be realised.