Does passport office always return old passports?

Hi All, I'm going to apply for renewal of my adult British passport as the expiry date is approaching. However, I would like to keep my old passport for some other application in the future to prove I am British Citizen for the last 10 years. Does passport office always return my old passport after the renewal is complete? I tried to search the answer in official gov site but I don't seem to see anything explicitly mentioning they will return old passports. Thanks, Jon