I disagree. The requirement is to be 50 years or older. The reason you state in the TM.7 is "retirement". This is also what is stamped into your passport by immigration.
I have had two full british passports for over 25 years now . Britain is one of the few that actually issues FULL second passports that are valid for 10 years. Many countries allow second passports but limit them to 1 or 2 years.