Remember that the patient is responsible for the self-certification for the first week using green form, SC1 . So for the first week the doctor will not be able to provide a NHS sickness certificate. In this case the doctor may occasionally be able to provide a private medical certificate but there will be a cost involved. The SC1 form is available from your employer or at the Post Office.
After the first week a certificate may be obtained from your doctor on request where you would need to be seen by your doctor.
If requesting for an extension of sick note this could be done without an appointment, though it is up to the doctors discretion.