The insurance probably depends on how much the annual cover is for, costs less for say £4k per year than for £7k.
I think the idea with the food is that they give you the amount they should be getting if that's their only food, and reduce it appropriately if you have mixed e.g. on the Royal Canin dry I use it says about 75g per cat per day, but to reduce that proportionally if I also give wet food. Mine aren't too greedy, so I can put wet down twice a day & they self-regulate with dry that's down all the time.