You must have missed the start of the show where he said it was all tricks. Remember it was a recording of a stage show. The audience are expected to see the start of the show.
From Wikipedia:
https://en.wikipedia.org/wiki/Derren_Brown
Brown does not claim to possess any super-natural powers, indeed his acts are often designed to expose the methods of those who do, such as faith healers and mediums. He makes clear in his performances that all of his apparent abilities, which manifest on stage/screen as feats of memory, intuition, mind-reading and control of other objects/people, are achieved through a variety of psychological means, such as hypnosis, suggestion, cold reading, misdirection, and showmanship, and in many cases he will often give detailed explanations of his specific methods during or after the performance. In some performances he also uses his techniques to explore issues of personality such as fears and motivation.