I think a crucial bit of context is that in the UK, many people who are seeking diagnosis as adults grew up in areas where there were no or very few child psychiatry services in the 80s/90s. In such areas only very profound cases would be referred to out of area services. Most people with neurodevelopment disorders in such areas were diagnosed with SpLDs like dyspraxia or dyslexia which could be diagnosed by community paediatricians, usually with evidence from educational psychologists and occupational therapists.

In fact the pattern is almost the opposite of what you'd see in the US where it would be hard to get diagnosed with a SpLD and e.g. ADHD was more widely recognised. But the rub lies in the fact that ADHD, ASD and many SpLDs have fairly high rates of comorbidities with one another, to the point where if you've got dyspraxia and no other diagnosable comorbidity, you're actually in the minority of people with it.