Trying to control Pornography on the internet is the equivalent of trying to prevent the sun to rise every day; it's doable if you do not want to face reality.
In reality and in the current economic situation, it is an impossible task to achieve for a number of reasons, most of which revolve around money.
The current scheme includes having an opt-fin form where someone fills in a few details and wait for a few hours before accessing the horny bits.
The first problem the government will face will be determining what consitute pornography and how to control it, something they tried to do in the 1970's but failed miserably.
Being global, the internet is not constrained by national boundaries, which makes things even more complicated.
This will also smack of censorship; How will erotic or "arty" websites be controlled, what about erotic Japanese manga, who will decide what pornography in the UK amounts to.
This leads to the second issue which is financial; if the onus falls on the ISPs to filter and control pornography, then we can expect a rise in broadband prices as they try to manage and control traffic to adult websites and pass on the cost to the end user.
The other bone of contention is whether the filters will actually work; people can easily circumvent any protection by using links available on social networks like Twitter or Facebook or even Google. Furthermore, anyone can get pornographic content from non web-based sources like newsgroups, P2P, forums or download websites like Rapidshare.
Loopholes will always exist and may eventually lead to what the Telegraph's Milo Yiannopoulos calls an acceleration of the nanny state rollout.