So there's clinics (doctors who can prescribe the cannabis), and pharmacies (people who can dispense / post out to you / collect payment for the cannabis).
Some clinics also have their own pharmacy, and prefer to handle everything in-house (that might be good for them, and can be convenient, but it's not always good for the patient).
So you'll go to a clinic (once you've decided which one) and their initial consultation / intake process covers getting medical records and determining if you qualify for being prescribed medical canna, and there are no red flag risks.
Every (reputable) clinic is going to want to check through your medical records, see copies of ID, proof of address, etc, because they are prescribing a controlled drug, you must have a qualifiying disability / long-term condition, and must have tried two forms of treament already. There will be other factors but this is the broad strokes of it from a patient point of view.
I'd say only use a clinic you can see listed on the page below on this site we're on now (
MedBud.wiki) since there are some dodgy places popping up.
Which clinics prescribe medical cannabis in the UK, and how much do they cost over time?
MedBud.wiki