A sick (fit) note is a legal document.
For the first week, you do not need a doctors fit (sick) note for any illness. Your employer may however require you to complete a self-certification form (SC2) which is available from your employer or on the HMRC website.
After that, a ‘fit (sick) note’ is required and must be signed by a doctor (GP or hospital) or some pharmacists (called ‘clinical pharmacists’) which are then counter-signed by doctors.. But you do not always need to see a doctor (or pharmacist) in person to get one. They look like this. There is more information on them on the NHS website here.
If your employer agrees, a similar document can be provided by a physiotherapist, podiatrist or occupational therapist instead. This is called an ‘Allied Health Professional (AHP) Health and Work Report’.
If you are self-isolating and cannot work because of COVID-19, you should get an ‘isolation note’ from NHS 111. You do not have to go to a GP.