As with most large corporations, BA have adopted the N.I.H. syndrome, which is now part of their corporate culture, and not just SOP's. "Not Invented Here" is simply a myopia to other companies' way of doing things, even in this extreme case where the a/c manufacturers have developed their SOP's in line with the a/c development.
Within BA, There are many other such NIH examples, but it is not really kind to label the company as Blindly Arrogant, so let's leave them to get on with it themselves.
BA must surely have considered the legal/litigation consequences of an accident occurring during their operations whilst following SOP's not standard or recommended by the maker of the a/c. ??