When you call at the Customer Service Point to collect your parcel please bring some form of ID, to confirm that the package is yours. If you are collecting for someone else, please bring their ID. We are quite flexible; a driving licence or bank card is easiest but a passport or work id is fine, as are bus passes. If the package has no name on it you will need ID that includes your address, either a driving licence or a household bill. Royal Mail is a bit behind the times and does not mention digital forms of ID but I regularly accept digital bank cards or student ID. While you are supposed to bring your P739 card I rarely make a fuss about people who have lost or forgotten it, but you will not get your package if you do not have ID.
(Disclaimer: this A to Z is not official Royal Mail advice, except by coincidence.)
Stay safe. Be kind. Collect your package promptly.