The most up to date links to the general guidance on COVID-19, social distancing and/or self isolation can be found on the homepage of Doctorlink.
You can also access the latest government guidance on Social Distancing here: https://www.gov.uk/government/publications/full-guidance-on-staying-at-home-and-away-from-others/full-guidance-on-staying-at-home-and-away-from-others
If you have been told to stay at home because of coronavirus and you need a note for your employer you can also use the NHS 111 service https://111.nhs.uk/isolation-note/