Continuity mistake: When Laura Knightly leans against Roy for the cameraman to take a couple photos, her face is even with Roy's. In the next shot she's above him so her chin is at Roy's eyes.
Continuity mistake: When Jerome pulls one of the disabled men out of his wheelchair, both of the disabled men on screen are in different wheelchairs to those they used in the scene where we first see them (when Roy files the police report).
Continuity mistake: Near the start when Moss runs and jumps onto the couch, his tie changes positions repeatedly between shots. In close-ups it is up on the couch arm and in all other shots it is under Moss.
Continuity mistake: Season 2, episode 5 "Smoke and Mirrors": Roy's lipstick alternates between bright pink and more of a peachier colour throughout the scenes.
Answer: It's a joke on the "118" numbers in the UK. 118 numbers were used for directory enquiries and adverts for them on TV always tried to give a way to easily remember one for a particular company. The fake ad in the show takes it to the extreme. People in the UK do not use "911", the point of the joke was that Roy had forgotten "999" while saying it was easy to remember.