You will not require a doctor's certificate for absences from work of less than one week.
If you need a self-certification it can be obtained from your employer or from the reception.
If you require a private certificate (as requested by an employer for absences of less than one week, this can be provided and will incur a fee.
General guidance on sick notes see the following link
If you need to send your sick note for ESA please see the following link for advice https://www.gov.uk/send-fit-note