If you’re off work sick for 7 days or less, your employer should not ask for medical evidence that you’ve been ill. Instead, they 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. We call this self-certification. The form is available from your employer or on the HMRC website.
https://www.gov.uk/guidance/ask-your-employer-for-statutory-sick-pay
Find out more about sick notes on the NHS website.