From Lasors:
An applicant who does not already hold a Night Qualification/Rating (Aeroplane) shall be given additionally at least 5 hours night flight instruction.
Before undertaking the Skill Test, the applicant shall have passed the associated theoretical knowledge examination, and completed all of the related flying training
So, it looks like that if you turn up at a school for a CPL without a night rating, the FTO should give you 5 hrs of night flight instruction before letting you loose on the Skill Test.
Is there some particular reason for wanting to defer the night flying until after the CPL training?