Fast-food is not affordable in the UK. It's substantially more costly than preparing your own food. There is no way that we could afford to eat fast-food more than a couple of nights per week; then we'd have to cut the quality of food on other nights considerably.

McDonalds for one adult costs about what we spend on a meal for 4 cooked at home (not including energy costs).

That said friends we have friends on benefits who eat fast food regularly. How they afford it I don't know. They have the heating on all day (with windows open, in winter) and things too, it's all a mystery to me.