Hi Alex,
In my case ( assume is the same for everyone) mya contacted my GP. My GP then got in touch with me and asked me to write a letter stating what procuder I was having, and pay £30 for private letter. A week later I had phone to call to say my GP wouldn’t write consent letter, and to make an appointment to meet with her. Made the appointment, we had a chat about why I was having the surgery( risk involved etc), and wrote the letter right there and then. Hope that helps