You do not require a doctor’s sickness certificate for any illness lasting seven days or less. Your employer may however require you to complete a self-certification form (SC2) which is available from your employer or on the HMRC website.
For any illness lasting longer than seven days you will need to see the doctor for him/her to issue a sickness certificate and for any subsequent renewal of the certificate. If your employer insists on a sickness certificate for 7 days or less a charge will be levied.
You can complete a .GOV form with this link