simply go to any of the supermarkets around London eg. Tescoes, Sainsubrys, Waitrose and look for the section where they put the mark down items. Maybe not all shops do this, but the ones I went to did.
Marked down due to being close to expiry dates or slow selling stock, but the food is absolutely fine.
You can get some incredibly cheap and nice meals / food eg. 1-2 pounds for a dinner. Then all you need is a microwave to heat it.
I ate very well for a week doing this for a mere fraction of what it would cost to dine out or buy takeaways.