Under current legislation a patient 'self certificate' is used for the first six working days (ie one week including Sunday) of any illness. The self certificate (form SC1) is available from the health centre, your employer or the Post Office.
After the first week, certificates are obtained as part of a consultation with your doctor. If you are under the care of a hospital doctor either as an inpatient or outpatient, you should ask the doctor in the hospital for your certificate.
Some employers insist on sickness certificates for illness lasting less than seven days. As this is not a statutory requirement, it is considered a private arrangement and a charge will be made for issuing a certificate.