So if you have arrived as part of the crew of the aircraft you can go for your medical. If you have arrived as a passenger to have your medical you need to quarantine first. The medical is irrelevant, it is your role on the incoming flight that determines quarantine. There is however an exception if you are arriving for medical treatment so if you have a medical problem that needs correcting for your medical to be issued you dont need to quarantine, although you wont be able to be an in patient in a UK hospital without 14 days isolation.