Starting My ADHD Diagnosis Journey

Hello, I'm 26(m) and have only recently started looking into a diagnosis. So glad to have found this sub! Apologies for the long post - it's a mix of getting everything off my chest and asking for some advice. I'll summarise at the end.
ADHD is something I knew nothing about until a couple of weeks ago but after speaking with my GFs sister, who has recently been diagnosed and realising we're very similar, it made me realise there might be a cause to a lot of my own behaviours that I've been beating myself up about for years. So the following day I researched as much as I could into ADHD, various charity websites, questionnaires, NHS information and realised that all of this material was like reading about myself.

After doing all my research I went and spoke to my GF about it, and cried like a baby; a big thing for me as someone who's regularly told that I have the emotional range of a teaspoon. Although I'm a long way from any kind of diagnosis, I feel like a weight has been lifted off my shoulders: maybe I'm not useless and unmotivated and maybe I can do something about it.

I'm now actively trying to get some kind of diagnosis and I'll be having my first phone appointment with my GP tomorrow morning. This is the part where I really don't know where I'm going and why I'm glad to have found this sub. I haven't found much information about what the process is for obtaining a diagnosis here in the UK. I also can't really use my GFs sister as an example as her family have paid privately to get past the waiting lists.

TL;DR: I'm fairly certain I have ADHD but I'm not clear on what the process of diagnosis is through the NHS. I'd appreciate if someone could break their diagnosis down into steps for me.

Edit: I got my referral :)