The following link can provide the most up to date information about the Coronavirus (COVID-19) and whether you do need to self isolate: nhs.uk/conditions/coronavirus-covid-19/
You first need to check this to ensure that your circumstances meet the requirements to self isolate for the certificate to be valid.
If the advice is that you do need to self isolate and you require a note for your employer then you will need to follow this government link, which will provide you with the certificate you need; you can complete this on behalf of someone else.
If you have to stay at home but feel well enough to work, ask your employer if you can work from home.
If you can work from home, you will not need an isolation note.
The Practice will not be issuing sick notes for Coronavirus.