The motivation, according to Cameron, is to protect children from inappropriate sexual imagery. Well and good, but apart from all the technical and legal difficulties, there seems to be a curious assumption that households with children will not request access to pornography. Or if they do, the parents will take steps to ensure the children are not viewing pornography...which puts us right back where we are now, with parents responsible for what their children see online.
Granted, it can be difficult to monitor children's internet access, but it is not impossible, and there are plenty of software programs and resources to help. The best solution would be if web pages contained content information, similar to movie ratings. This was tried in the 1990s, but failed. SafeSurf still has a website, but their last press release was in 2007. ICRA, the Internet Content Rating Association, became more popular than SafeSurf and was incorporated into Internet Explorer and Netscape Navigator, but it shut down in 2008. One of the key proponents of the ICRA has written a detailed paper on the challenges of web site ratings systems.
Meanwhile, the adult entertainment industry has set up its own system for putting a content warning into web pages. Unlike the thorough, detailed, and complex ratings created by SafeSurf and ICRA, the RTA Label is a simple identifier that tells filtering systems a web page contains adult content. However, participation is voluntary and the labels only work if a filtering system is installed. British ISPs could use this as part of their approach to filtering content, but if a household has opted in to pornography, and has children, the parents would need to ensure filtering software is installed...and we're back to parental responsibility again.
Cameron also announced new restrictions on content featuring sexual violence, such as portrayals of rape in sexually explicit films. The assumption here is that portrayals of rape are okay on TV and in mainstream movies, but substantial research demonstrates sexual violence in mainstream films has serious harmful effects.
Opting in to receive pornography online is hardly a censorship issue - it is no different than choosing a pay-per-view porn film or renting a DVD. It's hard to argue that the opt in program and new restrictions will cause any harm, but whether they will do any good is questionable. Pornography is frequently the scapegoat for a wide variety of social ills. However, an argument can be made that at least some pornography is hate speech and should be restricted. The sexualization of children and sexual violence are real problems that deserve more attention than just internet filters, but a little attention to what's out here is not a bad thing.