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) which is available from your employer or on the HMRC website.
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).
If you're sick and off work for more than seven days, your employer will probably ask for proof of your illness. Most employers ask for a fit note from your GP.
For more information go to https://www.gov.uk/taking-sick-leave
For information on Statutory Sick Pay (SSP) see the DirectGov website (where this information was sourced).