... but also should lead to criminal charges.
Indeed. There must clearly have been some kind of conspiracy here. The relevant group in Royal Mail's management would have quite quickly realised that there must be a system fault as more and more sub-postmasters/postmistresses came to their attention. But they kept telling the victims that they were alone. And the decision to prosecute must have been taken at the highest levels.
At the time, apparently, the Post Office had the power to conduct its own prosecutions. Perversion of the course of justice?