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 employers or on the HMRC website http://www.hmrc.gov.uk/forms/sc2.pdf
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.
For more information see the DirectGov website (where this information was sourced)