its the big metro cities that are ridiculous with food prices. I only live an 1 or so out of los angeles and food prices can be very reasonable depending on where you shop.
London isn't so bad as you only have to drive 30 mins from the city centre to reach the 'not so British' areas lol. Rice, eggs, chicken, beef, fish, veg, spices are dirt cheap.
You will always pay for convenience however, if someone deems it worthwhile to pay 150% of the price to avoid the travel/stress of travelling to buy groceries then it's all good.