Hmm, I foresee a few problems:
1. Proxy Servers - Has David Cameron ever heard of proxy servers? These nifty web services allow you to access any website from anywhere, so long as you can access the proxy server. Using the proxy server means that you can surf anything your heart (or other bodily organ) desires and there's nothing that the UK government can do about it.
Yes usage of proxy servers is low in the UK at present, but that's because the UK has no internet censorship. Just watch usage increase as 1 January 2014 nears. But these are difficult to use, I hear you say. Actually they're not but also when people are denied their access to porn, just watch the proxy server setup services spring up...
2. Misclassification - The existing web filters that I have used at work are poor at best. Misclassification is not uncommon and highly annoying when it occurs. A 1% error rate and I've seen much, much higher rates quoted, would mean many million of sites being mistakenly classified as porn and thus inaccessible from the UK. Imagine if your site had once had an image of Michelangelo's David on one page among hundreds on your site, for some web filter vendors that would be enough to brand your whole site as hosting nudity and thus porn. How would you get your site delisted? Could you? What would it cost you? How much business might you lose?
3. The thin end of the wedge - It's restricting access to porn today, in the interests of the children ("Will nobody think of the children"), what next? Bomb making sites? Radical Islamist sites? anti Islamist sites like Jihad Watch, The EDL website? UKIP? Once a government starts down the road of internet censorship, a precedent has been set and subsequent governments will be always tempted to extend the scope of the restrictions. Today there's no internet censorship in the UK, in 20 years we could be like China or one of the highly restrictive Middle Eastern states (not Israel).
4. Publicity - How much would these opt-in lists be worth? How much would The Mail pay for such a list, so that they could check on teachers who have opted in? How much would The Guardian pay to see which Conservative MPs have opted in? I presume that you could think of many, many more such examples.
Pornography filters for the UK, it's coming sometime... but should it and will it work?