because I'm not a UK citizen (yet), BUT YOU CAN!!!
Petition to ban plastic carrier bags
I realize that plastic bags aren't the worst problem facing the envirnment, but they are a problem, and one of the most easily fixed, at that. Pick up a couple re-usable carrier bags the next time you're at the supermarket, keep them in your car so they're always handy (if you drive) or by the back door. Plastic bags are just unnecessary. There's no excuse.