Surely the suspected criminals here are no less guilty if the law is changed after the fact ? They broke the law AT THE TIME. That's what makes the act criminal. What it is now is irrelevant.
The government might, in the same way they did for Alan Turing, offer a post-trial and perhaps post-mortem pardon if they judge the criminal acts to be non-criminal in the light of later laws. But the point is, the civil servants deliberately flouted the law as they knew it, and, unlike Turing, in the course of their work. That's what makes them, and their managers, unfit for office. They can't be trusted.