Wikipedia says the director wanted to use a camera that was more maneuverable because the movie is filmed with a lot of movement. Also it says that they filmed in downtown London without a permit, that they just like asked people to not go on the streets that they were filming. Having a portable camera helped with this. The camera that was used was the Canon XL1.

In addition to what dead said, there's an artistic element. The last scene switches to film after the characters have largely escaped the horror and made contact with organized groups (I remember a fighter jet or something?).