I find this M&S Buy on Board thing to be largely crap tbh. Coming back from AGP on quite a light flight you're understandably a bit peckish at 00:30... there was nothing dairy free (dietary requirements are very poorly catered for from this range) so that was my girlfriend going hungry for a bit while I necked down the gin.
Coming back the other day from AMS, I hear that they don't stock toasties that are advertised in the booklet because it'd take too long...
Some of the powers that be or at least somebody in BA will see all this but alas nobody cares what we think, clearly customers expect too much lol.