My guess: "no, but only because the government will introduce a clause saying it doesn't count so long as the ISPs are acting in good faith to comply with legislation".
(But the block page will probably hedge saying something like "our filter system indicates the page is likely to contain pornography" rather than "does contain", with a link to fine print explaining that false positives are inevitable . Or maybe you will just get a connection failure.)

But child pornography is already blocked, and I would be surprised if wrongfully accusing someone of hosting child pornography wasn't libel. I assume the much smaller volume means overblocking isn't currently a problem there. (And looking at the IWF FAQ, they don't block UK sites (they get them taken down).)