If you have been recommended to ‘self isolate’ by your GP or via Government Guidelines or are ill for more than seven days, your employer can ask you to give them some form of medical evidence.
You do not need a doctors Sickness Certificate or Fit Note 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 wesite.
Patients can get a Fit Note only if appropriate from a GP or hospital doctor. If your employer agrees, you can also provide evidence from someone who is not a medical practitioner such as a dentist, physiotherapist, podiatrist or occupational therapist instead.
For more information please see the DirectGov website.