There is now a petition against the proposed law on the UK government's number 10 website.
http://petitions.pm.gov.uk/Violent-Porn/
If everybody who signs it gets two more people to sign it, in a fortnight, the number will by far exceed the number of people who signed the original petition in favour of the law.
Edit: Initially, I rather optimistically projected that the number of signatories would increase exponentially, doubling each day, so that by now (5th December, Day 17 of the petition being up) we'd have thousands of signatures...
Unsurprisingly, that didn't quite happen, but signatures have been coming in at a remarkably steady ra