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.
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).
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.
You could also provide evidence from someone who is not a medical practitioner, e.g. 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.
You do not require a doctor's Fit Note for any illness lasting seven days or less. Your employers may ask you to complete a self-certification form (SC2) which is available from your employer. For any illnesses lasting longer than seven days you will need to see a doctor for a Fit Note. If, despite this, you still feel that you need a Fit Note from your doctor during the first seven days of your illness, please note that there may be a charge for a private certificate. No Fit Note will be issued without an appointment with the doctor.
For more information see the DirectGov website (where this information was sourced)