If you have been ill for 7 days or less, a sick note is not necessary. You can self-certify upon your return to work. Many employers have their own self-certification forms, but you can also download one from this page. Please print it, fill it in and hand it in to your employer. You do not need to see a doctor to self-certify.
If your employer insists on a sick note for an illness lasting 7 days or less, you can still request one from your doctor. However, please be aware that there will be a charge under these circumstances.