I can't speak for Horizon. However in the BCC Oracle delivered exactly what was asked for - several times over the years as the spec changed. Why are they to blame ?
The UK hasn't learnt yet (admittedly it's only had about 60 fucking years) that specifying and managing technical projects - of which IT is a paradigm - requires listening not to the people who want it in pink. But the people telling you it needs to be built this way and tested incrementally.
The end of HHGTTG ("OK smart alec, what colour would you make it ?") must have been used as a civil service training video since 1980.
So already the search for blame is going in the wrong direction. There isn't enough irony in the world here.
Fans of IT fucks will remember that the software for the Nimrod project ultimately doomed it to failure with GEC unfairly taking the blame. In reality the RAF never wanted it (they preferred AWACS) and at it's height the MoD were submitting 10,000 change requests a year. (This was the project with the disappearing scientists)