Monsters - The Smilers

Featured in the episode "The Beast Below," the Smilers are androids, mounted in booths all across the space faring version of Britain that left Earth in the 29th Century. They let people know what's good and what's bad. The terrified population keep them clean. Their heads can rotate to show three faces: happy, warning, or very dangerous indeed.

And if things get very dangerous, the Smilers can leave their booths and move around. They're actually androids, an extension of the 'starship's' security system.
bbcamerica.com

There are also the Winders, the police force of Starship UK, who were actually cyborgs of half-human and half-Smiler. Their heads would rotate around to reveal the malevolent version of the Smiler face (though with a lighter skin tone, whiter teeth, flat nose and brighter eyes). The majority of the time they would wear dark hoods and act as normal Humans, keeping watch on the population. They were under oath to protect the system of Starship UK.
tardis.wikia.com