You do not require a doctor's sickness certificate for any illness lasting 28 days or less [ From 10th December 2021 ]
Your employer may however require you to complete a self-certification form (SC2) which is available from your employer or on the website GOV UK Employees Statement of Sickness.
Statement of Fitness for Work - ’Fit Note'
The 'fit note' was introduced on 6 April 2010.
With your employer's support, the note will help you return to work sooner by providing more information about the effects of your illness or injury.
Check 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 fitness to work note.
Doctors issue fit notes to people to provide evidence of the advice they have given about their fitness for work. They record details of the functional effects of their patient’s condition so the patient and their employer can consider ways to help them return to work.
You could also provide evidence from someone who is not a medical practitioner, e.g. a dentist. Your employer will decide whether or not this evidence is acceptable. If your employer has any doubts, they may still ask for a medical certificate from your GP.
Other information related to the Fit Note is available below:
https://www.gov.uk/government/publications/fit-note-guidance-for-employers-and-line-managers/getting-the-most-out-of-the-fit-note-guidance-for-employers-and-line-managers
Fit Note Guidance For Employers