First of all, it is about blocking by default and opting out(as you mention yourself at least for O2 before editing it to 'opt in', furthemore different providers provide either in or out by default). Then if you look at the img you posted yourself at http://i.imgur.com/dWxORfJ.png you will see that it is not only about pornography but a dozen other things including areas such as 'Obscene and Tasteless'(?).

The government's job making a law of(and therefore enforcing) the above is easy to justify under the rationale that this thing existed for years(with a few specific ISPs). Now everyone will have to do it, and on top of that it will be the government that will be defining what is 'Obscene and Tasteless' as opposed to a mere ISP.

I understand what you mean too, but my disagreement genuinely has to do with me seeing that both filtering schemes are identical to each other and have the same purpose and effect. Both are opt-out and both do not have to do with pornography only. I sincerely do not see how these can be different.