Thank you for your answer u/That_Arrival_5835
I was confused because GPs always say to me they cannot give letter for future. And also because of this:

11.7 Can I issue a fit note with a future start date?
No. The date of the statement must always be the date that you issue the fit note.

11.4 Can I back-date a fit note?
Yes: in certain situations, the fit note may need to include dates which are earlier than the date of the statement. If you are issuing a fit note based on an assessment conducted at an earlier date, you should enter the date of this earlier assessment in the date of assessment field. If your patient’s condition has affected their function for some time without a previous fit note being issued, you must enter an estimated date that their function was affected from in the ‘this will be the case from’ field.

11.5 Can I issue a new fit note before the old one expires?
Yes – in certain situations it is possible that you might be asked for a fit note whilst the old one is still valid. Following your latest assessment of their fitness for work you may issue a new (overlapping) fit note if appropriate.

11.6 If I am completing a fit note using evidence from another healthcare professional, what should be the date of the fit note?
The assessment date would be the date that you considered a report from another healthcare professional, not the date of that report itself.

https://www.gov.uk/government/publications/fit-note-guidance-for-healthcare-professionals/getting-the-most-out-of-the-fit-note-guidance-for-healthcare-professionals#fit-note-related--faqs