UESPA, Star Trek's dirty little secret
It always made me smile when they'd be referenced in TOS, because in Ireland when I was growing up, "Spa" was an insult, so it sounded like Kirk and co were calling each other "You spa!". I was young.
I never heard it pronounced in acronym form in TOS, only referred as United Earth Space Probe Agency to a fighter pilot from the 60s, instead of Federation or Starfleet, to emphasize the UNITED EARTH part.
Besides, it's totally Wehy-Spah