You do not require a doctor's sickness certificate for any illness lasting 28 days or less. Your employer may however require you to complete a self-certification form (SC2) these are available from front reception or on the HMRC website.
Evidence that you are sick:
If your 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). It is up to your employer to decide whether you are incapable of work. A medical certificate (sick line), is now called a 'Statement of Fitness for Work', from your doctor is strong evidence that you are unwell and is normally accepted, unless there is evidence to prove otherwise. You could also provide evidence from someone who is not a medical practitioner, ie: 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. Where a Statement of Fitness for Work is arranged following a telephone consultation with the GP it must be collected by the person named on the certificate, from our reception.
For more information see the DirectGov website.