Yes you can be diagnosed with ADHD as an adult in the UK. I was provisionally diagnosed initially with ADHD and given Concerta, when it did absolutely nothing helpful and they did a full developmental history (the initial ADHD diagnosis was made after a one-hour appointment) my psych decided it was AS instead. I was already seeing a psych for bipolar disorder, not sure how difficult it might be to get an assessment if you're not already in the mental health system.