Continuity mistake: Roy closes his comic and throws it onto the desk. The camera flits back to Moss, then back to Roy, and Roy is holding the open comic again and repeats the same actions. (00:06:25)
Continuity mistake: Roy has scratches on his face from his and Moss' 'fight'. In the next scene, the scratches are gone. (00:15:30)
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.