@PineappleUpsideDownCake I can only speak to my own diagnosis experience (I ended up paying for a private consultation, and now have “shared care”, which means my private guy monitors my medication and reviews but the medication is provided through the NHS) but basically the diagnosis criteria is something like “significant impacts life in multiple areas, such as work, relationships, social, family, education etc”.
My school reports were always exemplary on the surface but it was pointed out to me that there was a common theme of getting easy questions wrong and hard questions right, lack of care on details, could do better if paid attention, and a gradual pattern of excelling when younger and then starting to struggle as work became more self-directed. So, for example, top of the class all through primary school, one unexpected B at GCSE, lower A-levels than predicted, was capable of oxbridge but didn’t get in, 2:1 instead of a first etc etc. You just struggle more and more as you don’t have an externally imposed structure. This goes for jobs too. I did not have a problem getting diagnosed but I know a lot of people gave had difficulties through the NHS as they were often not directed to the appropriate specialist. It’s potentially hugely damaging to gather the courage to persue diagnosis only to be told incorrectly “everyone does this, have you tried a to-do list?”
If there’s any way you could find the money for private, it’s worth it. Simply because I started though the NHS and they sent me a massive bunch of forms, I had to fill one out, then I had to post one to my parents , collect them all together and send them back. And no deadline! Nope. Not happening. My private guy does everything by email/online and sends nagging reminders and phone calls until you do it. He knows ADHD.