Please don't take what I am about to write as a criticism of you or a judgement of your experience, because that is certainly not intended.

First off, everything is dangerous, including vaccines. Yes, the dangers should have been explained to you. Actually, it would have been far better if your child had been tested first, as there are some fairly solid indicators as to whether someone will have a bad response. Nor can I think of any justification for having vaccines to go to Britain, as it has vastly superior disease management compared to many countries. Certainly, having vaccines several years early is totally unnecessary. If a child needs vaccines when in Britain, the NHS is more than competent to provide them. Well, ok, the severe defunding issue has made that less the case than it used to be, but I'd trust the NHS to make the correct call over any doctor who insisted on early vaccination on a pretext that doesn't exist.

Second, none of the symptoms described relate to autism. If a doctor told you that he had autism, based on those symptoms, the doctor was full of it. There is clearly a severe medical condition, I have no idea what condition it is, but it's not autism. The doctor who told you that autism isn't degenerative was sort-of correct (it is, in a sense, but not in the way that would normally be considered such), but utterly irrelevant as this is not related to any autism spectrum disorder.

Third, you were certainly done a severe disservice. What you described should have resulted in a battery of tests (probably starting with a check for meningitis, given the timescale and the fact that it was brain-related, followed by an MRI or CAT scan). Given that some of these conditions, such as meningitis, have to be caught incredibly quickly to avoid a very high mortality rate, it should never have reached the stage of behavioural disorders. If that had been one of these fungal or parasitic brain disorders, they're essentially fatal at that point. Doctors have a window of opportunity that is maybe one or two days.

(On a side-note, for some brain injuries and brain illnesses, the window of opportunity between injury and permanent brain damage or death is a few minutes to a couple of hours. After that, people can simply drop dead in their tracks. Doctors who have no concept of the importance of time or the importance of understanding should be kept to treating things that aren't time-critical.)

Of course, this doesn't happen much. Doctors abused diagnostics in America for reasons of profit and in countries that actually have adequate healthcare, the taxpayers aren't usually willing to pay for it. If you're going to run detailed tests on people when those tests are actually needed (and not in hindsight), you have to be prepared for the price tag. I think a child is worth it, I'm sorry that so many don't.

There's also the problem that parents do suffer from over-anxiety. I'm not saying you have, or have ever done so, but the fact remains that parents do. This is partly a product of poor education, schools don't cover healthcare as a mandatory core subject, but it is also a product of gross misinformation. That can include misinformation by doctors. The quality of healthcare information in the public domain is not good and if I had to rely on it, I'd probably be spending all my waking hours terrified too.

If your child has not had a high-definition MRI (sorry, low-definition scans will show nothing useful, they're not even good on injuries to boxers or American Football players where most of the brain has been turned into cheese), has still not been checked for antibodies or an immune response that really should not be there, and/or has not been checked for genetic markers or microbiome abnormalities that would result in abnormal responses to vaccines, I'd demand an explanation from your doctor and your local political representative. Things never "just happen", the universe doesn't work that way, either you have been fully informed of the exact chain of events or someone is not doing their job.

The one thing you can be absolutely sure of is that your child does not have autism and has never had autism. If that is the only thing you can be sure of, someone owes you an explanation. In-depth with no excuses.