Well, the first thing that came to my mind was "that's f**ing brilliant!". How Derren Brown slowly sneaked in and established control of everything is just impressive and in the same time shocking and disturbing.
What was a little bit implausible was the explosions at the beginning to mimic a meteor impact. OK, if the guy was already set up to belief that a meteor strike is imminent, a selective perception of the situation and confusion may explain why he oversaw the misbehavior of the "meteors". I am not sure if the difference would appear to myself if i would be in the same situation, but this was the first big clue that could make him aware that something different was behind the curtain.
Another thing is, as others already mentioned, how long this shock will last? We all know that sleep comes in rather quickly again. I am not even sure if he really grasped what was happening. The impression i had was the he constantly looked overcharged, somewhat disconnected from reality. I think, 2 days are way to less to let such a situation really sink in and cause major changes within the person. He constantly lived a shock and then, before real changes took place, he was released.
I am pretty sure, if he'd got the chance to let the true horror of such situation dawn on him, his true nature would be revealed, not just a person who act out of a shock.
But the theme also was very interesting. Why a meteor shower combined with a zombie apocalypse? Derren Brown sure have a good sense of humor, but how would Steve react, if this happens for real, now that he gone through a similar shock? Will he even belief it? I mean, if real comet fragments are going to hit the ground and explode in the atmosphere, a few fiery gasoline blasts cant keep up with such a kind of cosmic showdance.
All in all, a very revealing experiment on so many levels that show how easy people can be manipulated to belief everything. And if a simple man with a few cameras is capable of such a monstrous manipulation, how much more powerful other beings can be? True horror, even without a zombie apocalypse.