The reason why the NHS waiting lists are so long is because, as mentioned, demand has gone through the roof. I work as a Community Mental Health Nurse and the numbers of people wanting an assessment has gone through the roof. Tbh, I think the majority of those requesting an assessment do not meet the criteria for diagnosis, though many may well have other mental health conditions. Anxiety, for instance, may make many become restless, fidgety and unable to relax. I've worked in the field for over 30 years, and can think of only 3 people that have jumped out at me as possibly meeting the diagnostic criteria. One has since been diagnosed with it (on the NHS), another I think moved and as a result wasn't followed up by our ADHD clinic and the third us on the waiting list for assessment.
I know our ADHD clinic do not have a great deal of respect for these private clinics, for much the same reasons as the programme was saying...very brief assessments, and too keen to give the diagnosis. The treatment for ADHD is very, very powerful stimulants (speed, basically!) and they have extremely nasty side effects which is why they need a thorough medical prior to commencing on them and regular monitoring eg regular BP measurements, ECGs etc.
This maybe controversial, especially as I'm not a parent, let alone of a child with ADHD, but if I had a child with this diagnosis, I would not be happy for them to go on such medication. I think they're far too powerful for a child. But that's just my opinion and i know its not one that everyone agrees with.