...though I can't say I saw any of it in the (rather nice) Lamb Casserole I had on the SZG-LGW flight today

I guess using common ingredients for several different meals is one way to cut costs in the meals without (hopefully) sacrificing quality along the way - out of LHR the shorter routes like BRU sometimes feature the same salad for both meal options, simply allowing you to choose whether you want something vegetarian or meat/fish based placed on top of it...