You do not require a doctors 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’ from your doctor is strong evidence that you are sick and would normally be accepted, unless there is evidence to prove otherswise.
You may 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 aceptable. If your employer has any doubts, they may still ask for a medical certificate from your GP.
If you are under the care of a hospital, this Statement should be provided by them.
Please see link below for further advice on Sickness Certificates.
Advice from government website re sick leave