Suggestions and advice please

I have been smoking cannabis for a long time as it helps me with “self diagnosed” insomnia and ADHD. I have never spoken to my GP about this but I did seek help for my insomnia through NHS and received sleep therapy. It didn’t help. So I have continued to smoke cannabis to help me sleep. I would like to book a consultation with a clinic to find out if I can receive MC to help with my issues. I want to ask you guys what clinic would be a good place to start (I keep hearing about CB1) and what I would need to do. Is it a case of just phoning them and going from there? Do I need to speak to my GP first? Do I need to be officially diagnosed with ADHD first? Any advice would be great as I am a little nervous about making sure I do it the right way. Thanks