It is not necessary to see the Doctor specifically for a certificate in the first week of illness. Self-certification forms for the first six days of absence are available from reception, from your employer or the post office.
If a signed certificate is required to cover this period, then a private certificate is issued and a charge made.
After the first week, sickness certificates may be obtained as part of the consultation with the Doctor. Remember that if you have been in hospital, a certificate to cover any in-patient period must be issued before you are discharged.