Finally a sensible post in this thread.
BA know that Norwegian are trashing their yields - just check out the prices on BA flights to US destinations served by Norwegian vs. those that aren’t.
If they didn’t see the Norwegian model as a credible threat, they’d have ignored them and let them burn themselves out.
Maybe they’re helping Norwegian burn themselves out. Undercut on the same routes. Continue to charge a sensible fare on the remainder (which NAS don’t have the scale to fly, let alone compete on).