CAA Ref
You get a CAA Reference number:
When you book your first exam, when you get your Medical Certificate or if you apply for a FRTOL (Radio Tel Licence), whichever comes first.
You do not need one to sit the exams and the medical is not required beforehand.
If you hold a ICAO class 1 medical, you require an "extended ICAO renewal". Cost GBP 258.00