You do not require a doctor's sickness certificate for any illness lasting 7 days or less.
However, your employer may require you to complete a self-certification form (SC2) which is available from your employer or on the HMRC website.
HMRC WEBSITE SC2
Evidence that you are sick
If you are sick for more than 7 days, you employer can ask you to give them some form of medical evidence to support payment of SSP (Statutory Sick Pay).
However, this will also depend on your employer's company policy on sick leave (or sickness absence). This policy should tell you how many days you can be off sick before you need to provide proof of illness or a fit note.
If you need a Fit Note from your GP, you can request one by clicking the eConsult banner. The GP will review this request and issue the document, if appropriate. If you did not see your doctor for the condition a telephone or face to face consultation will be arranged.
A Fit Note is issued to provide evidence of advice given to a patient regarding their fitness for work. This records details of the functional effects of the patient's condition so that the patient and employer can consider ways to assist in arranging a return to work.
In certain circumstances someone who is not a medical practitioner may be able to provide evidence but if the employer does not accept this, a medical certificate from your GP will need to be sought.