(2) It shall be the duty of the Director[, subject to any provisions contained in the Criminal Justice Act 1987]—
(a) to take over the conduct of all criminal proceedings, other than specified proceedings, instituted on behalf of a police force (whether by a member of that force or by any other person);
& "conduct" includes bringing such proceedings to an end: section 15
We are lucky to have her and not a politician or an elected prosecutor or a supposedly French style independent investigating Magistrate.
She makes hard tough decisions on a daily basis, and no doubt gets it wrong from time to time, being human etc.
& as Lord Bingham made clear in R v Director of Public Prosecutions, ex p Manning  1 QB 330, once she has made her decision, she is answerable to the AG but save in exception circs giving rise to grounds of JR, nobody else:
We all have a right to express an opinion about her decisions, but not at the expenses of undermining the integrity with which this important office must be vested.