The first bizarre part to me about this fiasco is that accounting, as a discipline, is one that is designed to catch errors. Put it another way: it assumes errors will occur. This is why in shops, for example, you'll have manual stocktaking (ie let's verify what's in the store is what the computer thinks is in the store) and in any business you'll have reconciliation processes to find and remedy errors.
This highlights a key part of systems design. A key question you should be asking is: what happens when this fails? Note that's "when" not "if".
So something like Horizon should be used to flag cases for reviews. If a branch is found ot have a cash shortfall suggesting possible theft then there has to be a reconciliation possible to identify if the computer system was wrong.
Bugs happen too. How do they ever have confidence in the system and fix bugs if they can't determine if a given flag is a false or true positive?
But instead the system's output was taken as gospel with no possibility of verification. I'm of the belief that if you can't verify anything the system outputs, particularly for something in a discipline so used to verification as a concept, then that signal is worthless. The fact that convictions happened as a result of this is a crime. This is the UK and not the US so sadly that compensation will probably be limited to nonexistent.
As an aside, this is exactly why electronic voting should be outlawed. You need paper ballots (that can be counted electronically) as a verification measure. And the fact that we even have to debate that makes me sad.