The UK govt is testing the water to see whether the people will actually stand up for their online rights. Yes, its just an 'opt-in' for now but its unlikely that will be the last we hear of this.
Of course, this being the UK, there is little/no chance of that happening, and they probably know that. The net here is still viewed by the majority as a privilege, rather than a god-given right.
I wouldn't be surprised if the EU leaders were in on this. They may be considering an EU-wide filter post-Wikileaks and know that they are likely to encounter little opposition in the acquiescent UK population, start with the weaker links in the chain and move on.