a) http://news.bbc.co.uk/1/hi/england/london/6947744.stm
b) You are directly trying to impose your value over someone else's value, with the assumption that you are correct. One has to be very careful sure you are not just projecting your frustration at the littering. What they are doing is completely correct for their worldview, which has a boundary round about where their clothing is.
If someone litters at all then they probably haven't much of an awareness outside of their own egocentric POV and it seems quite unlikely that a single intervention by yourself is going to change that, unless they are a kid of less than ~ ten years of age.
I like to smile and pick the litter up as a gift. If they notice they notice, if they don't they don't.