To the left of Shadwell's newspaper advert is an entry pleading for the return of "Uncle Terry's" lost hat. It's one of many little homages to Neil Gaiman's friend and Good Omens co-author Terry Pratchett, who died before this series could be made. The hat frequently appears in his official author photos and is almost as famous as he is. That hat can also be seen in Aziraphale's shop, awaiting the never-to-come return of its owner.
Gabriel says "Human beings are so simple", putting down a copy of Mrs Beeton's Book of Household Management. It's a 2,000 page book of details about how complicated Victorian life is.
One of Adam's friends has the middle name "Moonchild". While this is implied to be a hippie name, it is in fact Aleister Crowley's name for the Antichrist.
Also alongside the "Join The Professionals" newspaper advert, and below the advert asking for information on Uncle Terry's lost hat and scarf (last seen in a bookshop near Soho), is an advert about a lost book. It isn't The Book (The Nice and Accurate Prophecies referred to in the episode title), it is The Colour of Magic. This is the title of the first of Terry Pratchett's Discworld books.
Archangel Gabriel names the Four Horsemen of the Apocalypse as War, Famine, Death, and Pollution. The biblical Four Horsemen included Pestilence rather than Pollution. Later they explain this by saying Pollution replaced Pestilence while comically muttering something about penicillin.