You do not require a doctor's sickness certificate for any illness lasting seven days or less, instead your will supply you with self-certification form (SC2).
Evidence that you are sick
If you are sick for more than seven days, your employer can ask you to give them some form of medical evidence to support payment of SSP (statutory sick pay).
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.
More information regarding Fit Notes can be found on the Government Website via the following links.