From the second to the third season, the series changed from being a distinctly British show, with the exception of Jon Hamm in Season 2 Episode 4, Black Mirror: White Christmas (2014), to featuring American actors in most main roles. This was because the first two seasons were produced by British production company Zeppotron, whereas the third, fourth and fifth seasons were commissioned and produced by Netflix, an American company.
Before the release date of season 4 was announced, Black Mirror's Twitter account had been liking tweets about the show that were posted on December 29 from the previous years, which is the release date of season 4.
Despite numerous references in different stories suggesting they all take place in the same universe, series creator Charlie Brooker has stated each episode is set in its own world (excluding possible sequels to specific stories).
Domhnall Gleeson, Natalia Tena, Jessie Cave and Kelly Macdonald have all had roles in the Harry Potter franchise. Gleeson was Bill Weasley, Tena was Nymphadora Tonks, Cave was Lavender Brown and Macdonald was Helena Ravenclaw, Ghost of Ravenclaw Tower.
Letitia Wright, Daniel Kaluuya, Hayley Atwell, Benedict Wong, Pom Klementieff, and Anthony Mackie appeared in the Marvel Cinematic Universe. And Anthony Mackie, so far, the only American actor from the MCU, that guest starred in Black Mirror.
During Season 3 Episode 2 - Black Mirror: Playtest (2016), when Cooper (Wyatt Russell) reaches the door in the house for the 'Access Point', Katie (Wunmi Mosaku) says, "Would you kindly open the door?" And later says that reaching the access point is them ensuring that he will follow instructions without question. This feels like a reference to the first Bioshock game where the main communication in the game, says "would you kindly" for each objective, and this is found out to be for the same reason as in this episode.