There are private clinics and private clinics though aren't there. Not everyone seeking a private ADHD assessment will be assessed with the same rigour and robustness so it is likely that some individuals seeking private assessment may well be misdiagnosed. Misdiagnoses happen in NHS assessments too all the time, but they don't profit from it.
Not sure if anyone remembers this news story from last year
https://www.bbc.co.uk/news/health-65534448 about some private clinics offering a very short online assessment to then provide an ADHD diagnosis. Note one of the clinics is called Harley Psychiatrists located in Olivia's fav place to get medical treatment, Harley St, London. And oh look, their assessment is only 45 mins and costs nearly £700 a pop. And they look cheap compared to other providers...
You can understand why private providers are filling a massive gap the NHS can't fill, the NHS waiting list is extremely long for an assessment and private clinics will be able to offer a quick turn around for a diagnosis and for some people that will be life changing. For some people though, they may receive a diagnosis that they don't realise actually doesn't apply to them where perhaps a different diagnosis would, which can be really harmful. And for a fraction of people with the funds to spare, they'll be able to answer in a way that allows them to be diagnosed with ADHD and receive a prescription.