In America they diagnose it so quickly and a lot of kids are medicated, some unnecessarily. I suspect the UK is trying to avoid that as the drug (Ritalin) isn't great for its side effects. Sadly it tends to "numb" a lot of people out which means, although it helps with focus it takes away people's natural personality
I am in America and have a child who was diagnosed with ADHD. Here is our process.
We talked to the school, we filled in forms, his teachers filled in forms, we visited the school counselor, we visited his GP, then we visited an educational psychologist, then he spent two 6-hour days with the ed psych doing a whole battery of tests.
She then went through the test results with us, and wrote a 15 page report with recommendations.
The process took about 3 months. By the end of it we knew several incredibly valuable things about DS, how his brain works, what he struggles with, and how to help him.
We gave the ed psych report to the school, then the school set up meetings including all his teachers and the deputy principal, as a group we came up with his 504 plan.
We hold a meeting each year, and have two meetings in years where he has major changes (like a move from middle to high school).
He is medicated, and it makes a massive difference to how he copes with school and life. It does have some side effects - mostly on his appetite - but it certainly hasn't numbed his personality in the slightest.
He also has many other adjustments in place, which also make a massive difference to how he copes with school and life.
The main difference is that he is much happier.
One of the results of this whole thing is we're all now aware that MIL has ADD - it explains the struggles she's had throughout her life with organisation, being on time, focusing on conversations or tasks. She's not medicated, but she has put in place many coping strategies without ever actually receiving a diagnosis.
I do now look at certain friends and colleagues differently now I understand more about ADD - I understand more what they're struggling with, and especially with colleagues I sometimes help them put in place systems to help them focus and work more effectively.