Honesty is the best policy....if you plan to give zero then say zero on the slip. This will send a message to whomever sees the donation slip and may possibly confirm that they are not on their own in resenting this rich U.S. based religion for asking folks from the U.K. to give them money. Also, you may not be the only one in your congregation who will refuse to donate. It could make quite an impression if there are many who refuse to give anything.
Also, saying you are going to donate a large amount makes you resort to dishonesty (which ultimately hurts you) and it may serve as an encouragement to those who view the donation slips, when they see that someone is giving generously.
If the donation slips are anonymous, perhaps you could enter a big fat ZERO and give a short but powerful explanation as to why.