As far as I can tell one would have to block pretty near every site and then release sites as they become demanded and approved. Can't see that being popular.
This is not something that ISPs can handle.
Apart from anything else, it's currently illegal in the UK to intercept transmissions, which is what the ISPs would have to do. Surely they would not do that...
Well, talktalk DO do that although they call it "virus protection" and apply it retroactively (AFTER the content has been served up) AND they parse it via a chinese company, so plenty of security and privacy protection there.
BT experimented with Phorm and was not slapped down, although Europe got a bit threatening about it until their "IT guru" changed and they decided it was not a problem after all.
Virgin almost tried Phorm but seemed to have better sense in the end.
Sky is a shambles anyway and would probably do anything.
So, even if it's illegal to intercept traffic (Deep Packet Inspection) that will not matter - in UK. I understand USA has been rather harsher, though.
Still, it would make it easier to present tailored advertising, eh? :)