The waiting lists on the NHS are awful - 3 years in our area, so GP advised we went private as it was a case of when not if we got the diagnosis.
It took about 8 weeks to get a private appointment, but that gave time to get all the questionnaires completed by school and external teachers (generally it's one teacher at school that does it, so if they only teach DC for a couple of hours a week they may not know them that well, so we got long-standing extra curricular teacher to fill it in as well), plus send pack of school reports, family history etc.
Psychiatrist gave us the outcome at the end of the assessment and started meds the following day.
It is horribly expensive, especially if you are also paying for the medication. Check GP will do shared care as we've moved to that and it's now free rather than £60+ a month.
Check ups are every 6 months once they are stable, can be more frequent in early months if they struggle with the meds.
They have been life-changing for DD and she is so much more focused and less fidgety on them, and big difference in grades.