Under government sick pay regulations, no doctor's medical certificate is required for an illness lasting seven days or less.
A self certificate form should be completed if the illness lasts between four and seven days (including Saturday and Sunday). These are available from your workplace or your local benefits office and some job centres and main post offices.
If the illness lasts for more than seven days then you do need to obtain a medical certificate from the doctor. If your employer requires a doctor's certificate for an absence of less than seven days, then a private certificate may be issued by your doctor for which there will be a charge.