Fit for Work Certificates
You do not require a doctor's certificate for any illness lasting five working days (or 7 including a weekend). Your employer may however require you to complete a self-certification form (SC2) which is available from your employer or on the HMRC website. You should refer to this link for more information
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).
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 Doctor.