There is a concept in Information Technology called "The Parallel Run". I don't think BA can have heard of it.
WHat it means is that you run the new system in parallel with the old system until you have assured yourself that the new system delivers what its supposed to.
Why the heck did BA switch a large chunk of its operations to a new and untested terminal with associated systems in one go?
Surely it would have been safer and far less disruptive, to have started with a few "real" flights and confirm that the wonderful systems worked as advertised, then gradually increase the load on T5 and reduce it elsewhere?
Oh of course! You need loads of staff to do it that way!