You do not require a doctor's sickness certificate for any illness lasting 7 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:
Evidence that you are sick
If you are sick for more than 7 days, your employer can ask you to give them some form of medical evidence to support payment of SSP (statutory sick pay).
It is up to your employer to decide whether you are incapable of work. A medical certificate, now called a 'Statement of Fitness for Work' (see below) from your doctor is strong evidence that you are sick and would normally be accepted, unless there is evidence to prove otherwise.
Statement of Fitness for Work - 'Fit Note'
The 'Fit note' was introduced on 6th 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.
For more information see the https://www.gov.uk/