You can self certificate yourself for the first 7 days of any illness. If your employer insists on a sick note to cover this period then I’m afraid the practice would have to charge you for a private sick note.
Any period beyond this would be certified by your GP after discussion with them. They are now referred to as a “Fit” note rather than sick note as the idea is to try and make your employer aware of what you can do rather than what you can’t . We might make suggestions such as reduced hours, sedentary duties only, unfit certain parts of your job etc.
If you need an extension to your fit note, please provide as with as much information so that we can ensure we can do it accurately . If we do not have sufficient information to explain your condition (such as a hospital discharge letter) It is likely that your GP will need to examine you so please do not assume that one will be issued particularly retrospectively .