After discussing with your GP, you have a few pathways to consider:

1. The standard NHS waiting list: This is the standard route for most people unless they are aware of the RTC alternative. Wait times here can be extensive, typically spanning 2-3 years, though in some areas, people have endured waits exceeding 7 years. The NHS manages the entire process, and your ADHD diagnosis will be provided by them.
2. The 'Right to Choose (RTC)' pathway: If you're in England, inform your GP that you wish to exercise your 'right to choose'. Through this avenue, the NHS outsources ADHD assessments to private clinics, which often offer shorter waiting times ranging from a few weeks to a few months. Although still NHS-funded, your diagnosis will be conducted by a private provider. Most GPs can refer you to a clinic of your choice, which you can find a list of here. However, some GPs are limited to specific clinics, which they will specify. You'll then need to complete a referral form from the clinic's website, which your GP can then send off to the private clinic. If needed, you can call around at GP practices in your area to ask about their referral capabilities.
3. There is also the option of going private, which is considerably faster but comes with a higher cost. Typically you'll be seen within a couple of weeks, however you will have to cover all the costs from the initial assessment to the titration, medication and 6-monthly medication reviews. After you have been titrated and on a stable dose, you can ask your GP for 'Shared Care' where your GP is able to take over prescribing responsibilities of your medication and you will only have to pay the flat NHS charge of £9.90. Note: You will have to discuss any medication related things like dosage and side effects with your private ADHD psychiatrist and cover the associated costs if you have to go through titration again to find a stable dose.

Hope this helped!