Illness lasting seven days or less
You do not require a Fit Note from a GP 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 the HMRC website.
Illness that requires more than seven days absence from work: Statement of Fitness for Work - ’Fit Note'
If you are sick for more than seven days, your employer can ask you for medical evidence of your condition to support payment of SSP (statutory sick pay). Your employer's policy on sick leave (or sickness absence) should tell you how many days you can be off sick before you need to provide proof of illness/a Fit Note from your GP. If you need a Fit Note, the simplest way to request one is via eConsult (click on the button above). Your GP will review this and either simply issue it for you to collect from Reception or, if the certificate is for a condition you haven't consulted about previously, possibly call you or arrange for you to come in.
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 GP.
For more information see the DirectGov website