A GP cannot make an Autism assessment or diagnosis, unless specifically qualified and authorized which is unlikely.
A GP may be able to provide original documentation, most cannot. *
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Also:
In England, Asperger's is no longer a separate clinical condition with its own diagnosis.
ADHD unlike Autism is not a permanent 'disability'.
An Autism diagnosis by itself does not entitle PIP.
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People with an Autism diagnosis can apply for an ENCTS Disability pass and then a Disability Railcard. In the past some authorities refused ENCTS applications based on an Autism diagnosis only. This should not be the case now.
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*Parents etc., keep every scrap of documentation from specialist, doctor, school, benefit claims, awards, etc., forever!