As an excuse to remove my DC from my custody because I’m not capable
If you have autism you have autism whether it's diagnosed or not. There are hundreds if not thousands of undiagnosed autistic parents in the UK today and at least thousands of diagnosed autistic people who are also parents.
If your parenting is not in question before a diagnosis, surely there's no need for it to be in question afterwards.
People have been sectioned for autism
Autism is not a mental health condition.
See this from Why would someone be sectioned?
www.rethink.org/carers-family-friends/brothers-and-sisters-siblings-network/get-info-and-advice/sibling-faqs/what-does-being-sectioned-mean
The Mental Health Act would only be considered if someone was very unwell and will never be taken lightly.
Someone needs to meet certain criteria in order to be sectioned. This criteria is that someone has to be suffering from a mental disorder of a nature or degree which warrants your detention in a hospital for assessment or treatment and that you ought to be detained in the interests of your own health, your own safety or with a view to the protection of others. (Mental disorder is a broad term that includes conditions like schizophrenia, depression, bipolar disorder or personality disorder).
I'm pretty sure that if autistic parents en masse were being sectioned just for being autistic, or having their children taken away from them just because they are autistic the media would be all over it. Certainly all the autistic bloggers and tweeters would be campaigning.
I think you need to do some more research into what having a diagnosis of autism means.