You only need a doctor’s note if you are unable to work and are ill for longer than seven calendar days. Your employer will provide you with an SC2 self-certificate form for shorter periods of illness.
Sickness Certificates for periods of less than 7 days should be signed by the patient themselves, in the form of a self-certification Form (SC2), available from your employer, GP or online at HMRC using this link
A statement will not be issued to a patient until they have been off work for more than 7 calendar days. DSS certificates of incapacity are free and GP discretion see Non NHS Services.