CSLM10211 | FAQs: employers questions and answers: employee leaves: what do I do when a borrower leaves?
From HM Revenue & Customs · Collection of Student Loans Manual
Suggested answer
Enter 'Y' in the Student Loans box on form P45, indicating that student loan deductions should continue if
You have received instructions from HMRC to start making deductions, (a Start Notice or a form P45 with a 'Y' in the Student Loans box)
And
You have not received a Stop Notice
Send the leaver information to HMRC in Real Time Information and provide the employee with the P45 Parts 1A, 2 & 3. The new employer will then continue making deductions where appropriate.
If you have received a Stop Notice leave the Student Loans box blank on form P45.
Find more information about what to do when an employee leaves on gov.uk,