Pepper's line to War, "I believe in peace, bitch", is a quote from the Tori Amos song "The Waitress". Amos and Good Omens co-writer Neil Gaiman have been friends for many years and frequently reference one another in their works. Amos' version of "A Nightingale Sang in Berkeley Square" is used as the ending music.
Gabriel says "God does not play games with the universe" echoing Albert Einstein's famous criticism of quantum theory, "God does not play dice with the universe."
The brass band in the park is playing "Lazing on a Sunday Afternoon", from "A Night at the Opera" by Queen.
Dick Turpin was a notorious highwayman who was active in the 1730s.
When Crowley throws himself, Adam, and Aziraphale into the peaceful ¨neutral¨ sky, the angel and demon arrive with their wings out. Aziraphale's are pointing upward and slowly point downwards, then rise to level with his shoulders and back upward again, where they remain. Crowley's start off pointing downward, then flow and stay upward for the rest of the scene. A sort of shorthand for how far they've come in their respective character arcs.