"About 95 per cent of UK internet users are thus shielded from the darkest corners of the web by their ISP"

is simply not true - the IWF do not blacklist sites hosted in the UK, instead they issues a takedown notice. There is inevitably some delay between them issuing a notice and the ISP removing the content. If they blacklisted then it'd be effective far more quickly so you it's certainly true to say that 100% of Uk internet users _can_ access childporn. The most likely reason they don't do this is because they're worried over liability in the country they're based in...

_If_ you accept the arguments for blacklisting it needs to be open, not by some unelected Quango.