If you know that you're going to be off work for an extended period then basic courtesy (and retaining a good relationship with your employer) requires that you should inform the employer as soon as possible. (If, say, you're about to have major surgery, which requires a long recuperation period, that might well be quite a way ahead of your absence, rather than once it's started).
However there's no need to submit a
formal 'fit note' (which is what 'sick notes' have become nowadays) until after 7 days absence. Indeed, it could cost money to do so because doctors are allowed to make a charge before that period is up.
https://www.gov.uk/taking-sick-leave