SPM10310 - Statutory Sick Pay (SSP): forms and guidance: SC2 - self certification
For SSP purposes, an employer is not entitled to ask for a medical certificate, signed by a doctor, for the first seven days of absence from work due to incapacity. The employee is only required to notify the employer of the incapacity and provide self-certification of their incapacity, in writing.
Form SC2 is available for this use, or the employer may provide employees with their own form or accept a letter giving the information they require in order to consider SSP entitlement.
Copies are available from GOV.UK