and the post office management had no interest in proving otherwise
they should be going after the management
In this case it should have been very easy to provide evidence to override the presumption that the Horizon system was working correctly. That this didn’t happen seems to have resulted from a combination of bad lawyering and shameless mendacity on the part of Fujitsu and the Post Office.
Don’t get me wrong — the whole thing is a giant scandal. I’m just not sure if this particular presumption of UK law is the appropriate scapegoat.
Defense had to prove that only one Horizon/Fujitsu accounting software was buggy and the whole prosecution falls apart e.g. If John's Horizon/Fujitsu accounting software has bugs then Peter's Horizon/Fujitsu accounting software most probably has bugs too.
One case where defense did get access to the code (FST developed by NYC) led to discoveries (https://www.propublica.org/article/federal-judge-unseals-new...) that led to it being retired from use.
I’d argue that some kind of weak presumption along these lines clearly makes sense and is probably universal across legal systems. For example, suppose the police find that X has an incriminating email from Y after searching X’s laptop. Are they required to prove that GMail doesn’t have a bug causing it to corrupt email contents or send emails to the wrong recipients? Presumably not.
