At my surgery (NHS) if you require a ‘sick note’ within 7 days of your first days absence from work, you are required to pay £5. However after the statutory 7 day period the note cost’s nothing. I was under the impression you are allowed the first 7 days as self-certification, thereafter a certified doctors note is required.
I would imagine Brockenspectre is right in saying the situation would be different if you were to attend the surgery as a non NHS patient.