Billboards over public green spaces like Hyde Park. These would be live feeds of people in these spaces, informing them about the problem, and that they are sat on the solution.
Using 0.001% (25,000) of the coffee cups thrown away in the UK every year, create a life-size garden installation to highlight the scale of the problem, and educate the public about the solution.
Casings that make bins in public places look like take-away coffee cups with information printed on them.
'Keep Off The Grass' signs with information about the problem will be pushed into people's front lawns instead of their letterboxes.