CSLM10212 - FAQs: employers questions and answers: employee leaves: t/p is leaving before start date on start notice
Suggested answer
Even though you have not yet started to make Student Loan and or Postgraduate Loan deductions you should enter a 'Y' in the Student Loan box on the form P45.
This will instruct the new employer to begin making deductions.
File away the Start Notice for your own records.
You can find more information about what to do when an employee leaves on gov.uk.