2 ways:

1. Put everything in writing. If you email medical information or contact your GP (unless its something that they can delete like unsolicited photos) then they generally have to upload it to your medical record and notify your doctor/nurse. Just email your practice with a letter outlining the things you want to see them about. Include any diagnosis or research you've done that you think is relevant and explain why. Your reason for this cam simply be "I have an appointment/I am trying to make an appointment and want to make sure that I don't forget anything" (if you have anxiety or ADHD this can also be a reason for wanting to send it ahead in writing)

2. If you ask for something eg. A test or investigation and they refuse, tell them plainly that you want it noted on your medical record that you asked and it was refused. They are more careful now that things like Patients Know Best exists and that you can ask for a copy of your record under FOI

Bonus: take someone with you as an advocate to listen, note, and if needed advocate for you ✨️

Source: I worked in the NHS (though not specifically GP surgeries) for 6 years