Wait, when did the P45 become part of the screening process in the UK. I have my P45 and the only parties that have asked for it are banks and credit agencies. If a recruiter asks for a P45 off he bat I'm marking them as spam.
Well, if tried a strategy of not telling the new employer direct what your salary is, or worse, lying, they will quickly see when the P45 is passed onto their payroll to sort out your P45. Which is why it is better to simply state you promised the agent that you would discuss the salary with them, and would then focus on discussing the specific about the role with the employer.