IME, Kosher meals ex-LHR/LGW tend to come from the same supplier (I forget the name) but on the occasions I've had them (AC and GSM Y - hence my 'single supplier' hypothesis) were more plentiful and edible than the regular offerings.
ex-YYZ, I tend to get a bagel with light cream cheese from Tim Horton's.
ex-LHR, the Boots "Meal Deals" can be a great value option (IIRC, you choose a sandwich/pasta, chips/snack and drink, and pay no more than about 3.50UKP). This might be the best bet for the OP if dietary restrictions need to be considered.