Re: big shops vs small shops.
This also includes lunchtime visits to Tesco Express, Boots, etc. which is very common in cities - particularly London. So I expect this would account for the skew, along with "city-people" being more likely to eat-out.
It is worth noting though that it's actually often more cost effective to buy your groceries every day (assuming you're purchasing at standard big-shop prices) because you'll only buy exactly what you need for the day - rather than speculative purchases. This means your food will be a little fresher and less likely to go bad due to not using it when you expected to.