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 SSP (statutory sick pay).
Your employer can ask you to confirm that you've been ill. You can do this by filling in a form yourself when you return to work. This is called self-certification.
If you're sick and off work for more than seven days, your employer will probably ask for proof of your illness. Most employers ask for a fit note from your GP Practice.
However, this will also depend on your employer's company policy on sick leave (or sickness absence). This policy should tell you how many days you can be off sick before you need to provide proof of illness or a fit note.
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 Practice.
Returning to Work
Discuss with your employer about returning to work first. They may be able to offer additional support to help you start back. You can return to work whenever you are ready, even if your sick note is still valid.
You do not require a note from your GP Practice to say you are fit to return to work. If your employer has concerns, they should request a medical report from your GP Practice in writing. More information can be found at https://www.nhs.uk/common-health-questions/caring-carers-and-long-term-conditions/can-i-go-back-to-work-before-the-end-date-on-my-fit-note/.