You can go to 111 for fit note for a 7 day duration - https://nhs.uk/isolation-note
After 7 days you will need to contact your employer - here is proforma letter for your use:
Get the latest NHS information and advice about coronavirus (COVID-19).
Find out about the main symptoms of coronavirus and what to do if you have them.
Get a test to check if you have coronavirus, understand your test result and find out what to do if you're contacted by NHS Test and Trace.
Advice about staying at home (self-isolation) and treatment for you and anyone you live with.
Advice for people at higher risk from coronavirus, including older people, people with health conditions and pregnant women.
Advice about avoiding close contact with other people (social distancing), looking after your wellbeing and using the NHS and other services.
'Your COVID Recovery' helps you to understand what has happened and what you might expect as part of your recovery.
Government information and advice.