I put "Prime Ministers" plural because it happens to all of them at some point, whether it's Cameron, Brown, Blair....
It's just not relevant for a Prime Minister to know the exact cost of a basket of a specific food. So many (as can be seen in this thread) will try to make cheap political capital out of it.
I usually do the shopping in our house and I know that the bread I often buy costs about £1.30. Don't have a clue about a sliced value loaf though!
Have a go at this. I scored 2!
http://www.bbc.co.uk/news/magazine-24345784