It's a negligible freedom. The CPS can always choose to seize control of any prosecution and, if it sees fit, discontinue the prosecution. So at most they can choose to ignore a prosecution unless either the prosecutor or accused bring it to their attention specifically, and if that happens they're obliged to actually do something (though that might be explicitly taking no action, it's distinct from ignorance because there's a process they have to follow by law to decide what to do).

This is an important defence - every Post Office defendant could have chosen (or rather, their lawyers knowing the law could have done so) to flag their case for this process and force the CPS to decide what to do and as you say it's conceivable that in the most egregious cases that would have stopped it †

There are definitely flaws in this system, I just have more confidence in it than I do for the American system. I didn't know Ross Anderson was dead. He did some really good work.

† We know it wouldn't have always worked because there are Horizon casees where the CPS are the listed prosecutor. Jurors weren't the only people who gullibly accepted that the Computer is infallible, but at least the CPS were not employees of the executives who knew this was bullshit and have largely escaped any consequences.