Getting assessed for ADHD through the NHS can take years because of how under funded mental health services are and going private could cost £600+.
Look into 'Right To Choose'. Its basically NHS outsourcing mental health services to private companies. All you need to do is get your GP to refer you to a specific psychologist (you can find an email template on their website). The whole process usually only takes two or three weeks.
AFAIK 'Psychiatry-UK' is the only place that does it and its only for England.
spoiler
Yes, its another example of Tories privatising the NHS from the inside but... healthcare pls.
Students can also speak to your university if you want to avoid using private healthcare companies but only a few Unis have qualified psychologists to assess ADHD. Most just put you on the NHS waiting list and you most likely wont get a diagnosis until after graduation.