CSLM10012 - FAQs: employers questions and answers: new employee: P45 has a 'Y' in the SL box - t/p is not a borrower
If your new employee’s P45 has a 'Y' in the box, you should start taking deductions from the next available pay date.
If your employee does not know which plan or loan they’re on, ask them to check in their online account. If they cannot tell you, use Plan 1 in your payroll software until you get a student loan start notice (SL1).
You can find more information on the gov.uk page.