This episode is based on the Philip K. Dick short story "The Hood Maker," originally published in 1955 in the magazine Imagination.
The picture seen at around 50 minutes in, of the couple with the hoods over their heads is "The Lovers" by René Magritte, painted in 1928.
Agent Ross's apartment is filmed in the 'Rowley Way' estate, off of Abbey Road in North London. It is also a location in 'Kingsman' (eggsy's estate). There is also a view of the long garages that run underneath the estate. The exterior shots are filmed from the Western End, the majority of shots in Kingsman were from the array of platforms and ramps that adjoin the eastern end of the long, curved estate to a row of shops.
"The quick brown fox jumps over the lazy dog" contains every letter of the English alphabet. It's used to preview Font Faces to show all of it's main characters at once in a real world example sentence on PCs.
Holliday Grainger and Richard Madden both appeared in the 2015 film 'Cinderella' and BBC's Lady Chatterley's Lover.