In "Tomorrow and Tomorrow and Tomorrow" we are introduced to James Kirk, captain of the UEFEnterprise. This prefix is visible on the dedication plaque when La'an arrives on the bridge.
Curiously, later in the episode when La'an asks Kirk where he was born he says the USSIowa.
What gives? Was the Iowa was a cargo ship? Maybe in this timeline, "USS" stands for "United Shipping Service." Or perhaps at some point, UEF ships did use the prefix and they changed it because the wanted "Earth" more clearly in the identifier.
Maybe their space fleets are all that is left of those specific countries? Maybe they were maintained in service with the vague idea of being a "government in exile" (or "society in exile") before eventually uniting as "United Earth", but purely in space.