"Seeking autism diagnosis for my child"

I need help.  My child got referred to NHS Ayrshire and Arran by our GP for an ASD diagnosis.  He has been rejected from assessment and support due to them thinking there is not enough evidence for co-existing mental health problems. They refuse to diagnose ASD anymore unless they can also see a comorbid mental health problem, which I think is unfair and discriminatory. As they have refused to even assess us, they refuse also to look for the evidence themselves. 

We home educate and don't receive special services, so there are very few professionals in our lives to provide corroborating evidence. I have already provided my own statements/evidence for both ASD and mental health but this seems not to be taken into account.  The rejection letter does not give a possibility for appeal.  I think the refusal to put a child on the waiting list for ASD diagnosis despite me giving clear evidence of ASD is wrong and the requirement for comorbid mental health condition is wrong. 

ASD is a disability in its own right and they are refusing to even consider us, for reasons unrelated to ASD.  This is only going to have negative consequences, as they want to put my child into situations where he becomes traumatised, before they will agree to put us on the waiting list.  The resulting trauma from withholding diagnosis now, in a timely fashion, will mean we will hit the mental health services much later in life when we will have bigger problems that could be prevented if the diagnosis was provided now.  This will ultimately be much more expensive for society and this particular service.  Please help.