The adolescents teaching each other how to get around filters - that is exactly what ends up happening. A few years back, Turkey had a pretty long ban on Youtube. Countless sites for tweaking browser settings specifically geared for Turkish Youtubers emerged. There was even a cartoon by a prominent artist that depicted the PM declaring his agenda: "We won't lift the ban until we have 70 million DNS experts."

Hey, at least it will produce another generation of tech savvy Internet users! Maybe we'll get some more engineers out of it.

&> The people who think it works get placated*

That is the whole point. Mr Cameron has just offended a group of Tory supporters, some of whom are rich and/or influential, with the gay marriage thing. In order to placate them ready for next time he needs their influence and/or campaign donations he is taking on this issue that the same subset care about.

If the controls are not particularly effective then all the better for him. The people he needs to placate will be placated for a time and the people that only care because they want access to porn will stop caring when they discover this won't stop them accessing porn (so he'll not become someone who has taken away some of their freedom). True free speech fighters will stay on the case of course, but there are so few of them that it'll not be a massive concern (and most of them will admit they have bigger fish to fry), many of the people say they really care about free speech are just not wanting to admit that they really care about porn.