Invalidated insurance

Who decides could be the person themselves: 'I think my condition is affecting my driving at night, or in heavy traffic' for example, or from what people who know you well think, or the GP or advice from a professional (eg Autism support worker).  The point is that you are given to believe you may be affected.  As with many of the conditions it is particularly vague and a bit o a minefield to navigate.

There are a suprisingly long list of ailments that you are required to inform the dvla about from physical to mental below is a link listing them and it's long

www.gov.uk/health-conditions-and-driving

There are also issues with travel insurance and house insurance.

Who actually decides whether ASD affects your driving or not apart from a case when a doctor diagnoses a patient as unfit to drive?

I don't know any examples but if your autism affects your driving (and it is not inevitable it does), then it in all probability would invalidate your insurance if you drove without informing both the DVLA and your insurance company.

The following is my opinion and interpretation of the law, it is not necessarily how a solicitor, magistrate, judge or court would see things.

The Highway Code (in so many words) says you must not drive if you are unfit. I believe 'unfit' in this case would not just include illness  (which autism is not) but also a condition which made you unfit to drive.  The Gov website says you must inform the DVLA on the appropriate form if your autism affects your driving.  It does not say you must inform the DVLA if it does not affect your driving.  This coupled with undiagnosed autism would make it difficult for the DVLA  to know if you were involved in an accident as a result of your autism, but you could not use autism as either a reason or excuse in the case of an accident - doing so would automatically say, and being an admission, that you were driving while unfit, which would be doubly bad if the DVLA had not been informed previously.

I do not believe that someone who is autistic would be refused a licence or insurance or be prosecuted merely for driving and being autistic.  In all probability they would be if they were stopped by a police officer for poor driving of a car while knowing their autism affected their driving adversely or had an accident as a result of their autism. 

But there would also have to be some outward mannerisms or an admission of autism - autism does not manifest itself in a physical way so it would either have to be you that told them (perhaps as a reason) or someone, perhaps in the car with you actually brought it up by saying you were autistic.

There are co-morbid conditions that you must tell the dvla about even if you consider your driving is not affected, but they don't automatically bar you from driving either, such as bipolar disorder.  Autism is under the heading 'must inform dvla if it affects your driving', and this applies to other conditions such as anxiety, depression, post traumatic stress and even eating disorder.  And as well as advising the dvla you would also have to advise your insurance company.

It should also be noted that if your doctor considered you unfit to drive due to your condition, (s)he could advise you to notify the DVLA and if you refused to do so would be obliged to inform them without your permission.  Continuing to drive without informing the dvla when your driving is affected could result in a fine of £1000

The following link shows the vagueness of the government's position:

https://www.gov.uk/asd-and-driving