While I agree there are some sites that "should" be blocked, where does the line lie? We already have the situation where torrent sites are now blocked, what about Russia today or other non european news sites? Sites of organisations and groups opposed by the UK government?
None of this is a new issue, we've always had the means for making judgements on where that line lies, and checks and balances for scrutinising those judgements. I don't see the fundamental difference between a newsagent not being able to sell top-shelf material to under 18s and an ISP verifying that an adult has OK'd delivering adult content to a household.
It's also not about what is blocked overtly but monitoring that takes place. The keeping of logs for 18months by ISPs (against EU law IIRC), monitoring of Internet traffic and the accessing of email accounts by government forces, many of which are questionably legal currently (which is why the security services and Cameron are trying to force through the "snoopers charter" again).
I understand you are very authoritarian so we will probably never agree on this but for many of us that would prefer personal freedom to th occasional chance of terrorism (or our kid "accidentally" stumbling on to a "dodgy" website).
You're right we won't agree on this because I'd hate to live in a totally free society. We live in an overcrowded island and crime, whether it be terrorism, child abuse, low-level anti-social behaviour, whatever, just makes everyone's lives more miserable. Not having total freedom to do what we want at the cost of those around us is a good thing imo.
As already mentioned this filter is the thin edge of the wedge that is very easy to bypass. I'm guessing every year 9 student now knows how to change the DNS setting on their personal internet device (computer/smartphone etc.). It was like that when I was at school (we would always bypass the filters) and I doubt it's changed. Unfortunately the younger generation are almost always more tech savvy than the generation before, it's a losing battle that never seems to be understood by politicians. The politicians who are invariably worried about new technology they can't control 100%.
No it's not the thin edge of a wedge - that's a slippery slope fallacy