You don't need to book in advance. Every month a session is held for around 9 days and you can go there without booking. You just need to register in the morning and tell the ladies at the entrance what exams you want to take.
As for the medical, I did mine at the military aeromedical center and you don't need to book it either, you just need to be there around 7/8 in the morning to register and you'll have your certificate around 2 in the afternoon.
Military Institute of Aviation Medicine