If you have been off work due to sickness for less than 7 calendar days and your employer requires a self-certification form but cannot provide one to you, you can complete a printed form and submit this to your employer.
Access a Self Certification Form
When do I need a sick / fit note?
7 days off sick or less
If you're off work sick for 7 days or less, your employer should not ask for medical evidence that you've been ill. Instead they can ask you to confirm that you've been ill. You can do this by filling in a form yourself when you return to work. This is called self-certification. Information and the form for self-certification
More than 7 days off sick
If you're off work sick for more than 7 days, your employer will usually ask for a fit note (or Statement of Fitness for Work) from a GP or hospital doctor. Fit notes are sometimes referred to as medical statements or a doctor's note.
For more information please visit;
When do I need a fit note? (NHS information)
The fit note: guidance for patients and employees (GOV.uk)