I can't say for certain but I have watched a lot of Derren Brown over the years and the guy is compelling to watch whether you actually believe any of it's real or not. However in tricks like this 1 the magicians usually use a method known as "forced choice" basically there is only 1 true thing that the "subject" could ever choose. In the case of Derren Brown he is more of a performance psychologist than anything and he really understands people. In a lot of his tricks such as this he spends time looking for "suggestible people", people that he can easily manipulate then he spends a lot of time following them around and subliminally planting images days or weeks before the show. It obviously is not 100% guarenteed to work so he chooses multiple subjects and I guess just by the rules of probability it does work on some and these are the ones you get to watch when the show goes out, Alot of what I just explained is actually from Derren Brown's own admissions.
There are a lot of Derren Brown's episodes on Channel4Player where you can see some of his admissions first hand.