Metro shops are more expensive than full supermarkets and while it might be pence per item, it really adds up over a full shop. Independent convenience stores even more so.
I was in London last week and there was a great convenience store that I used for a few things, but my God, it was expensive. I paid 79 p for a pint of milk and they only had pints. At home we pay about £1.30 for 4 pints and that's not the cheapest because we get it from M&S, Co-op or Asda or Morrisons Farmer's milk, which is more expensive so extra money goes to the farmer.
Just getting everything from the local convenience store isn't going to be an affordable option for anyone on a budget, so then you'd need to look at getting bigger shops delivered or bus/walk to somewhere cheaper.
Obviously for most in central London, or the middle of other cities, a car is rarely an option or actually needed due to parking, insurance and the fact that you have better public transport.
In other places people generally have cars because in a lot of circumstances, public transport is impractical and takes far longer than the car, so it's worth it and makes life a lot easier to have a car instead.