This is disturbing, and I can't really imagine how they want the ISPs to pull this off.
What counts as a porn site? Who will evaluate it? If I have a photo sharing site and some douche uploads a picture of a naked lady, does that count?
I'm ok with the child protection and all that crap, that should be parent's responsibility to begin with, but I think this is not the answer. They should at least ask everyone about this.
I really feel this is heading towards internet censorship in the UK.