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.
If you are unemployed and self-employed you can self-certify using Form SC1, this is available from the Job Centre.
If you are unable to return to work after the first seven days you will need a "Fit to work note" signed by a doctor.
If employers insist on a sick note during the first week of illness this will be in the form of a private note for which there is a charge to them, following a written application by the employer with the patient’s consent (i.e. declaration and signature by them).