In this trick, Derren Brown makes Simon Pegg forget the present he wanted and then makes him want something else. He gives an explanation in the end, suggesting he used some sort of NLP manipulation technique.
Click to see Deren Brown perform "BMX Bike" on www.youtube.com
There is a much easier way of accomplishing this effect than Derren suggests. You simply exchange the paper with a wish on it!
Instead of complicated psychological manipulation, he probably used some sleight of hand and his pickpocketing skills. He replaced the real wish (BMX bike) with a fake wish (a leather jacket). I am sure it is easier to fake someone's handwriting than to mess with his memory.
Which explanation do you find more plausible? It is up to you to decide.