Five years after leaving BA, I worked for an ad hoc charter company. I wrote an Excel spreadsheet to do our loadsheet. It was not certificated so, of course, a written loadsheet had to be completed before departure but it sure helped at the late planning stage.
I'm not offering excuses but TBH the ability to complete manual load sheets (which FWIW could have been done) is a bit irrelevant in the context of what went on on Saturday.....As I think has been mentioned more than once amongst other things there was simply no way of getting passengers checked in and bags to the aircraft side once the system failure happened. Rightly or wrongly there wasn't the manpower available to revert to a fully manual "paper" system and/or fallback to moving checked bags from check in to aircraft side by hand, and to be fair just this once I'm pretty sure these days BA isn't alone in taking that approach to manning levels, though I'd agree BA may almost be alone in having a failure of this magnitude...