Even if you don't give your new employer your p45, once your previous employer gives your details to the revenue, your new employer can get them. The employer can also request a certificate of tax credits, not just the employee. I have done this on many occasions, not to find out what the person was earning, but I work in the construction industry, where most of our employees are paid net to gross, so it's in our best interests to make sure all of the information is correct.