Seems like software failures are a common occurrence in british banks and airports. Perhaps a culture of corner cutting, and calling it agile, might be the cause?
Speaking as someone who (for a time) worked in the IT department of British Airways, the culprit is really just ancient systems that aren't well maintained, along with institutional and industrial pressure not to improve them or upgrade them. For example, I worked as part of the team that managed the software that allowed pilots to submit flight plans. Any upgrades had to go through multiple weeks of reviews and test…
For context: Imagine a crew room, with an old yellow looking windows 95 machine as the only "IT service" and only GSM-speed (or no reception at all) mobile internet in the area. The machine doesn't really work, or is on a dialup connection (yes this still exists).
These are the cases where we use the phone, or write the plan manually on a form and fax that. The phone has a high risk of mistakes, because you're reading lots of numbers and codes so it's easy to misunderstand one.