So North Korea has won?

It would do a lot more good to promote parents taking care of their children. No matter how noble the goal censorship is the wrong tool.

If Cameron has evidence that online pornography is corroding childhood he should stop concealing it. Such an important finding requires immediate public consideration and not disclosing this evidence of corrosion is disgraceful. Porn, like "violent" video game content, has been expanding in access and quantity for years now so showing a direct link to increased violent crime, sex crime, teen pregnancies and such should be fairly easy. Yet these indicators have remained stable or declined during this period of porn and gaming hyper-expansion. So we desperately need to see this new compelling evidence of Cameron's.
Cough it up ya wanker.

According to the Victorians, nude statues corrupted children. Then it was French postcards and "adult" photographs. Oh! The poor children! Millions of children were "corrupted" by finding prints in Daddy's nightstand. Then it was Playboy. I am sure all the 70-somethings will love to learn how badly they were corrupted by that. (Of course there were worse things than Playboy at the time, but it was considered mainstream and cool at the time.)
Then the VCR came along, and suddenly you could see actual pornographic movies at home. In... let's see... the 70s? So those who grew up with porn movies are now... running the governments!
But wait! Even before VHS, here in the U.S. you could actually watch X-rated movies at some drive-in theaters! At least around here you could. So the 50-and-60-somethings are all corrupted too!
Let's face it: the arguments of censors have never panned out. It's about control, nothing else.
Sometimes it's control by powermongers, sometimes by people who are so afraid of their own shadow, they are honestly concerned that others will be corrupted by the mere sight of it. But their irrational fears have never translated into reality. So my message to them is: grow up, and lighten up. Your attempts at control do nothing but damage.

Websites today use robots.txt to tell search engine crawlers what not to index on the website.
Add robotx.txt to any website addresses to see what files and directories they disallow for search engine indexing.
It takes a lot of work to keep an updated master list of all porn on the Internet but many porn sites are openly willing to identify themselves as porn.
Why not augment robots.txt to allow websites to (optionally) categorize content for search engines crawlers so that it doesn't put the effort on a central authority?
The robots.txt file would look like
User-agent: * Disallow: /api Disallow: /login Sitemap: http://website.com/sitemap.xml Adult: /
The advantage for porn sites for doing this is it would make it easier for people to find content when they are looking for porn and the advantage for people attempting to build parental control platforms or filter content from search results is that most/many adult websites would self police removing a lot of the required manual effort.
There is no free speech issue because it's entirely optional so everybody wins.

The internet is full of porn and other extremely objectionable content, I'd prefer to block my children from seeing it. That is censorship and I fully support it. However I don't really have the tools to do that, except by surfing with my children, which is impractical. It's ridiculous to suggest it! The world is full of examples where we require the world be modified to make it safer for children. We have playground speed zones and child labor laws and age limits for cigarettes and dirty magazines. The internet is just another part of the world, what makes it exempt? NOTHING.
I have used K9 freeware web blocker, but it is not perfect. Google, for all their billions of dollars, doesn't seem to do any rating for their web pages or youtube videos. I am happy to have a central agency perform blocking, that works for me. Everybody should be crystal clear on the concept, no adult will be giving up any freedoms in order to protect children. The internet has not been used responsibly, so it needs to be regulated. With freedom comes responsibility, collectively we haven't been responsible enough so therefore the freedom of use should be modified. Not curtailed - just modified.

@Grunchy
When an individual is forced to actively state that they want to view porn then they are giving up a freedom. Does it not make more sense to have opt-in filters rather than opt-out filters? You still get to protect your children, but you won't be expecting every other adult in the country to also do the same.
There's also the very real threat that this is just the first stage of censorship. It may be pornography right now, but it could be any number of other things in the future.

@Grunchy go regulate your own Internet and leave other alone please.
This is very well worded article and I have to agree with all of it.
I would like to add another aspect. Sexuality is a very private matter and requiring people to announce it to the government is a very shameful intrusion of privacy. This is not tolerable in my opinion.
If government would really care about people then it would provide tools to satisfy people like Grunchy that they could use in their households instead of applying a very questionable filtering to the whole nation.
I must say that the situation of the human rights in the UK goes darker and darker.

@Grunchy are you sure your children are not watching porn or not even seeing nude pics. the more restriction u keep the more they will search. After govt. applies the censor they will search and get to know about proxy server, then what will u do. And it is not hard to visit censored sites.
my personal experience - after my dad started to sneak, i searched Google 'how to hide internet activity'. that's it.

@Grunchy Schools and libraries are usually already filtered and for your home OpenDNS offers public DNS with free parental controls (FamilyShield).
It really isn't the responsibility of the Government to filter the entire Internet based on age and it isn't even technically feasible.