The BBFC's Dark Knight Summary, FYI

DANGER DANGER! HERE BE SPOILERS!

Just looked this up, and they seem to agree with my assessment (hehe - as if it was that way round!) see what you think.

THE DARK KNIGHT tells the story of Batman’s continuing war on crime and in particular his personal battle with the psychotic Joker. It was passed ‘12A’ for moderate violence and sustained threat

The BBFC Guidelines at ‘12A’ state that ‘violence must not dwell on detail’ and that ‘there should be no emphasis on injuries or blood’ and whilst THE DARK KNIGHT does contain a good deal of violence, all of it fits within that definition. For example, in one of the stronger scenes, Batman repeatedly beats the Joker during an interrogation. The blows however are all masked from the camera and despite both their weight and force; the Joker shows no sign of injury. There are also scenes in which the Joker threatens first a man and then a woman with a knife and whilst these do have a significant degree of menace, without any actual violence shown they were also acceptably placed at ‘12A’. In the final analysis, THE DARK KNIGHT is a superhero movie and the violence it contains exists within that context, with both Batman and the Joker apparently indestructible no matter what is thrown at them.

THE DARK KNIGHT also contains some special make up effects that whilst clearly not real, have the potential to be moderately frightening.
(Quoted From the Original BBFC Website listing)