No certificate is required for the first three days of any absence due to illness.
If you are absent for between 4 and 7 days due to illness, your employer may ask you to complete a self certificate form SC2; they can supply this to you or you can download from http://www.hmrc.gov.uk/forms/sc2.pdf
If your illness results in an absence of 8 or more days, you should arrange an appointment with a doctor and request a sickness certificate. These are free of charge. The Practice Nurses cannot issue sickness certificates.