This is the article about the Panorama episode and it doesn't mention any 20%.
https://www.bbc.com/news/health-65534448
I would say the episode in general was fairly misleading. There's no possible way that the journalist could have got the NHS appointment "blind" - because the waiting lists are years long. The NHS assessor knew that he was doing it for a programme. I don't doubt that they followed the correct procedure, but they are human and bias matters hugely. The assumption going in was that this journalist probably does not have ADHD. Whereas the private clinics have the opposite assumption.
Probably there ARE issues with some private clinics and certainly some of the bits of the appointments shown on the programme were concerning but the overall message of the programme seems to be (as you have taken) that ADHD is massively overdiagnosed and is a scam and not a real condition.
BBC said in their response to the many complaints about the programme that this was not the story they were intending to tell. They are merely questioning the efficacy of private clinics. They also explained here that the doctor assessing the journalist was fully aware of the angle of the programme, which makes him even more biased, compared with a doctor who perhaps knew this was a journalist but did not know that they were reporting with the angle of "private clinics diagnose too easily".
https://www.bbc.co.uk/contact/complaint/panoramaprivateadhdclinicsexposed
The thing is that the impression this programme gave is that these private clinics are handing out excess diagnoses. Whether or not that is true, it doesn't seem like there is a significant effect on the diagnosis or prevalence rate, so the idea that ADHD rates are being inflated doesn't seem to be true. Since that episode of Panorama was released, there has been this very detailed investigation/report (Independent ADHD Task Force) commissioned by NHS England. They also point out a problem with private clinics, BTW. Under "Executive Summary", the point about whether there is under or over diagnosis currently are point 5 and the point about clinics is point 6.
https://www.england.nhs.uk/long-read/report-of-the-independent-adhd-taskforce-part-1/
Quotes from report:
5. ADHD under or over-diagnosis in the UK?
England and the rest of the UK have much lower service recognition and treatment rates of ADHD diagnosis compared with other European countries (e.g. Norway, Denmark, Spain). Recent data show a very high level of under-recognition and under-treatment of strictly diagnosed ADHD, with significant inequalities in access to care (e.g. minority groups). For example, for those who meet strict diagnostic criteria for ADHD, fewer than one-third of children have contact with any type of mental health service, and only 15–25% of adults and children obtain pharmacological treatment. This means that demand on services is very likely to continue to rise.
There is also concern by some about potential over-medicalisation and over-diagnosis and a lack of regulation of ADHD service providers as this has been reported in some parts of the world.
Some also raise concerns about ADHD self-diagnosis based on information from social media. However, currently there is no good evidence on what percentage of those waiting to see a clinician have self-diagnosed ADHD using social media and eventually meet or do not meet ADHD diagnostic criteria after a high-quality assessment.
We only know currently that in England, recognised rates of ADHD are lower than the expected prevalence of ADHD.
[From Appendix 2: Evidence Summary]
1. What is the prevalence of ADHD and is it increasing?
In an English population sample of 2 to 19 year olds the ADHD prevalence was 3%; there is no evidence that the prevalence has increased since 1999.
Meta-analysis of global studies suggest an ADHD prevalence in children of around 3–5%. Geographical location does not influence prevalence significantly so these estimates can reasonably be applied to a UK population. There is no evidence of an increase in the number of children in the population meeting criteria for ADHD diagnosis over time.
For adults, meta-analyses suggest a global population prevalence for ADHD of 2–3%, which could be reasonably applied to a UK population.
2. What is the current level of access to clinical support and is there over or under-diagnosis of ADHD in England/UK?
While English health service records have shown increased recognition of ADHD in both children and adults from 2000 to 2018, the administrative prevalence is reported as 2.55% in boys and 0.67% in girls. In adults it was 0.74% in men and 0.20% in women. As the actual population prevalence in children is 5% and in adults is 2–3%, under-recognition of ADHD remains a problem in England.
Medication rates: English health service records also show that prescription rates of medications to treat ADHD have risen from 2000 to 2018. However, only 0.2% of the 7,655,931 [NHS patients included in a research database] received ADHD medication prescriptions.
Furthermore, the latest data from prescribing studies show that only 25% of children and 15% of adults with ADHD received pharmacological treatment, with regional rates across the UK varying up to 12-fold. The evidence from randomised controlled trials is that 70–90% will benefit from drug treatment so we are under-prescribing ADHD medication in England.
My comment is long enough but feel free to read the report if you want to know more about the impacts and why it is considered a useful diagnosis by the vast majority of specialists, whatever a handful of psychologists think.