National Autistic Society: 

https://www.autism.org.uk/ 

https://www.autism.org.uk/advice-and-guidance

The PDA Society - https://www.pdasociety.org.uk/

In many cases, NHS patients in England have the legal right to choose where to have their NHS treatment. This is known as “Right to Choose” and can often include who carries out assessments for conditions such as autism. If you or your child are facing a long wait for assessment and wish to find out more about Right to Choose, further information is available on the NHS's website and from the National Autistic Society.