Sick for less than 7 days
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.
Sickness for more than 7 days
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).
If you're sick and off work for more than seven days, your employer will probably ask for proof of your illness.
Medical certification is a legal document and your GP and/or Practice can only offer this if consultation allows.