The Co-op have a feedback page on their website (click "ethics in action" and then "have your say"), where I've just written the following:
I've come here to say that "Christian Voice" don't represent the majority views of Christians, and that many Christians are painfully aware of the damage that organisations like "Christian Voice" are causing to the reputation of Christians, and to relations between Christians and the wider community. Christians are a much more diverse group than "Christian Voice" would have you believe, and I'm glad that you're taking this stand against prejudice and bigotry. Thank you.
Regrettably, Christian Voice don't have a feedback page.
However, at the bottom of their press release, they've helpfully included the phone number and email address of their National Director, Stephen Green.