In our health board area, there is no provision for adult ADHD/ADD diagnosis as it is done by educational psychologists in schools. If you're an adult you have to rely on the dreadful mental health team. I was diagnosed as Borderline/Emotionally Unstable Personality Disorder by a psychiatrist despite only meeting the self-harm criteria. I was told that I couldn't have ADHD/ADD because I don't have hyperactivity. Second opinion is merely another psychiatrist from the same department reading the notes of the first.The diagnosis for ADHD and ASD is by answering loads and loads of questions. There is no blood test or whatever. Through the NHS you need a GP referral and the process will involve a specialist team of some kind, depending on how it works in your area. An adult patient would mainly tell the professional themselves. Many threads back Jack talked about making her friends and family write down their views about her behaiour or something- I'm sure this would be part of the process.
An educational psychologist employed privately can diagnose both- I didn't know this until I started working outside of the state sector. You could definitely read up on either condition and answer the questions to present as having either condition. A friend (never diagnosed) got ADHD medication from a private clinic in London and did everyting over the phone/ internet. In my jobI work with teenager disgnosed by a mixture of psychiatrists and educational psychologists. I am sure most people working in this area are ethical, but in my experience it is very very easy to get a diagnosis if you have money to throw at it.
I can understand why people who can afford turn to the private sector. I can't.