In my experience LHR-YUL/YYZ/YOW flights have always had the standard old chicken or beef choice. There are two chicken dishes and two beef dishes, of the four only one of them is any good (the stir fry beef).

I tried the vegetarian meal once (a good strategy on BA and most airlines, you get your food faster and it's hard to screw up a pasta casserole) but my experience agrees with yours and the other poster. It seems they have one sandwich which satisfies all the constraints: it's a vegan, low glycemic, bland, low-lactose tofu sandwich.

Honestly the happiest flight I ever took was when I ate at Chez Gerard before taking the flight and then passed on the airline dinner. Chez Gerard isn't as expensive as you would think and the food was actually quite good. And if you have to waste time somewhere in the terminal you may as well spend it eating

Alternatively stop by Pret-A-Mager and pick out your selection of fresh sandwich or salad satisfying your dietary constraints and bring it on the flight. It'll be a million times better than whatever AC would have.