Is 4chan the perfect Pirate Bay poster child to justify wider UK site-blocking?

I genuinely do not understand where how the idea of building a total surveillance police state, where all speech is monitored, can even as much as seriously be considered by an allegedly pro-democracy, pro-human rights government, much less make it into law.

Also:

Step 1: Build mass surveillance to prevent the 'bad guys' from coming into political power (its ok, we're the good guys).

Step 2: Your political opponents capitalize on your genuinely horrific overreach, and legitimize themselves in the eyes of the public as fighting against tyranny (unfortunately for you they do have a point). They promise to dismantle the system if coming to power.

Step 3: They get elected.

Step 4: They don't dismantle the system, now the people you planned to use the system against are using it against you.

Sounds brilliant, lets do this.

The thing you have to understand is that the average Brit wants and possibly needs the government to tell them how to live their lives. It's a completely foreign paradigm to the average American, though alarming "progress" has been made on the American front as of late.

Average Brit here - we do not like this and the way politics here has been so tumultuous has shown the general public are sick of this behaviour too.

Tumultuous in what way? There's so little distance between Conservative and Labour today that it really doesn't matter who's in power.