Thereby saving BA the extravagant cost of around a tenner per meal element not consumed.

I'm not one for excessive BA bashing, but this is ludicrous penny pinching on a first class ticket. It's not even vaguely acceptable. Quite what was going through the mind of the spreadsheet jockey who had this idea is anyone's guess, but it was probably a KPI on catering spend rather than customer experience. You can do a lot with not a great deal of cost to really impress people if the food is abundant.