When you start the job, the retail employer will automatically register you as an employee with HMRC. It's possible that the tax office will allocate some of your personal allowance to the new source of income though the rule of thumb is that the personal allowance usually stays with a pension as that is likely to be paid for the rest of your life.
You could ask the payroll people at the new employer to put you on a BR code whereby they will deduct tax at 20% and the tax code on your Police pension may be left alone. Alternatively, on the day you start, ask for the PAYE reference for the new employer, call the tax office on the number here
and ask them to make a note that you would like a BR tax code issued on the new job as it is only seasonal and that under no circumstances do you want your main tax code messed with.
If all goes to plan, at the end of the tax year, you may be overpaid as some of your personal allowance will have been unused so you can ask the tax office to review your liability for the year and to repay any overpaid tax.
If you find that some of your personal allowance has been used against the retail job, call the tax office and have a BR code issued at that point.
I hope this helps but let me know if you have any further questions.