McDonald's is messing with our heads a bit in this creative, British campaign that features prankvertising and some attention to detail (or lack thereof). Basically what's happening here is that McDonald's is trying to tell us that people can't concentrate on anything else when the Big Mac is in close proximity. Unlike the recent trend of daring ad stunts, and the million-calorie sandwich itself, the prank element here is pretty benign and quite effective. A couple on the street asks for a pair of different, unsuspecting folks to take a photo of them. As they prepare for the photo to…