Matt, not sure we've answered the questions fully for your purpose, a novel .
They would charge dangerous driving but further charge death by dangerous driving when the victim died. It does not matter when the victim dies; it could be years later but the Crown Prosecution could say that, if a very long time elapsed, that it was not in the public interest to charge death by dangerous. The defendant would have been tried and sentenced for the first offence before then, and on the basis that the victim was in a coma, so the sentence would be severe anyway. (They would go on if the dangerous driving causing death was particularly bad; the maximum, after all, is 14 years). The CPS might think that nothing much was gained by further prosecution, but they would be influenced in that decision by what the family of the deceased thought and what they perceived the public thought.
There is a new offence of "Causing Serious injury by dangerous driving" contrary to section 1A of the Road Traffic Act; the section has been inserted by an Act of 2012, to come into force on a day to be appointed, but I can't presently say what that day is (or was). That will be the appropriate charge for a driver who puts someone in a coma, but this nicety will not matter in a novel unless the action is set very recently !