I wonder how the airline would respond to a letter pointing out the error of their ways. It's worth baring in mind that from where dieticians and nutritionists stand at the moment, a diabetic meal is probably one that contains carbs with a low G.I.and avoids sugars, rather than low carb. Type IIs who choose to lower their carb intake are in a minority. I know several type IIs who wouldn't even consider giving up their bread/pasta/potato/rice, they just keep taking the tablets.
I was just looking at BA's offerings in advance of a trip to Mexico next September. You can choose from Lacto-ovo-vegetarian, Vegan vegetarian, Asian/Hindu vegetarian, Muslim, Kosher, child meal, baby meal, gluten intolerant, low fat, low salt, low calorie, low lactose and diabetic for which they say: "Does not contain sugars, syrups, jams, cakes and chocolate, unless they're specific diabetic varieties to help you manage your blood sugar levels". I suppose that's a start but considering the varieties they do I'm sure they could manage a low carb diabetic meal. Normally I've just left the bits I'd rather not eat, often thinking a bit of extra veg would be nice. I'll have to see what they say.