I have just sent the following email to warner bros along with an leaflet from the RSPCA website attached. I am furious! 
Good afternoon
As an avid Harry Potter fan and a wildlife & conservation degree
student I was horrified today to read about the release of 1000
balloons in London by Harry Potter actors to promote a competition.
I respectfully draw your attention to the attached file and url
regarding the impact balloon releases have on our environment and hope
that you will cease to use this irresponsible method in the future.
Marine Conservation Society: Balloon Releases: Pollution Factsheet
Yours sincerely