When you stop working for an employer, they will give you a P45.
A P45 is a record of how much they paid you in that tax year, and how much tax was deducted.
Your P45 will look like this
If, for whatever reason, you don't have a P45, you may have to complete a P46 form. The information you provide on this form will determine how you will be taxed.
Your P46 will look like this: