drosophila, it depends on your point of view. If you have more than £100 to spend on food per month, then why would you want to stick to such a tight budget? It's hard work and it does limit your choices. Naturally, if we had more money to spend, we would spend more on food - not a vast amount more, but we wouldn't be so budget conscious and count every penny.
Not sure if I understand your last question, but if you mean what proportion of your income do you spend on food, it's like this:
Income: £880
Rent: £500
Council tax: £60
Household bills: £200
Therefore: £120 left for food, clothing and petrol.
Hopefully we won't have to stick on this budget for long!