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.
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 sick pay.
It is up to your employer to decide whether you are incapable of work. A medical certificate can be issued if there is a medical reason for your absence from work and if required this can be sought via a routine telephone appointment with a GP.