When in Edinburgh, David Tennant (Crowley) uses a variety of Scottish accents, including his own. When in London, he maintains the character's regular accent.
When the camera zooms in on the laudanum, the label says it is guaranteed by CMOT Dibbler. "Cut Me Own Throat" Dibbler is a recurring character in Terry Pratchett's Discworld series of books.
Gabriel picks up a books and reads "In the beginning... it was a nice day. All the days had been nice..." This is the first line of the Good Omens novel.
When Crowley accelerates his car with the Angel in it, it speeds towards some colourful mountains and a lake. The colourful mountains are a reference to I Know Where I'm Going! (1945), according to Neil Gaiman's blog. In the lake, there's something that looks like the Monster of Loch Ness.
The end credits (of this Edinburgh themed episode) are played with bagpipes.