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) when you return to work, which is available from your employer or on the HMRC website.
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). Proof of your illness typically comes from a ‘Fit Note’ which you can ask from your GP.
For more information see the DirectGov website.