GP's point is that it's not "the police" accused of editing the statements, because these were private criminal prosecutions (a unique feature of the british legal systems) in which the Post Office acted as the prosecutor.

ie it's not a matter of trusting that "officers of the court" did this, because the prosecutor was not really an officer of the court, they were acting as a private citizen or (in this case) a representative of a private corporation. You are trusting that some rando working their 9-5 did not unduly "tweak" the words of the witnesses that are used as evidence to prosecute you.

If it was the Post Office editing Fujitsu's statements, who better than Fujitsu staff to confirm as a trial witness what was in the original pre-edited statements?