BA has signed deal to sell M&S sandwiches to economy passengers from January 2017. This applies to short-haul flights only and replaces the free "meal" currently provided. BA customers have reported a downturn in quality/quantity of late and the apparent introduction of low cost airline practices.
Waitrose apparently declined. Various questions have been asked about claiming Sparks points on purchases and whether the refund policy applies. As one cynic put it, the slogan of "To Fly To Serve" should now be changed to "To Fly To Starve".
Not that I fly BA these days (they don't serve Birmingham) but I'll stick with Boots Meal Deal or something made to order in the departure lounge.