PAYE61050 | Employment maintenance: create employment: employee withholds P45 from new employer
From HM Revenue & Customs · PAYE Manual
An employee may not hand Parts 2 and 3 of form P45 to a new employer because they want to keep details of the previous employment confidential.
You should tell a taxpayer who raises this point that withholding form P45 means
The benefit of the cumulative process will not be given
Any refunds normally received with the pay during the year will be delayed until after the end of the year
For further information see action guide tax80003.