It seems like a good idea in principle, but will fail epically.
The BBC report mentioned a couple of people who had used google to find such images. BUT once you've googled it once and found a site with lots of pictures/videos, you don't need to google it anymore, you can just go straight to that site. Meaning that this ban will be useless in stopping the worst offenders, who would have found such sites years ago. And others that are part of paedophile rings (which are worse than just individuals) probably didn't use google in the first place, they have a website for their own ring and share pics/vids with their friends and possibly email each other aswell. This new legislation won't stop this at all.
One of the big problems is, the obvious words will be blocked, but they'll still find a way round it, by inventing new words for it or using new code words, and that's if they even use google to find these pictures/videos, which I suspect the worst people don't.
It's not quite the same but .... My biology teacher googled 'blue tits' as in the bird, during a lesson. Because of the second word, the results were blocked and it had obviously been flagged by the computer and the head of ICT came in to see what was going on ! It was completely innocent and something relevant to our lesson, but was blocked. In the end, she googled the latin name for them and she got what we needed for the lesson. Something similar will happen in real life. They might not be able to use the obvious words which are banned, but they'll find a way around it, to get to the same results.
The problem with this ban on certain words and how search engines work is that it will end up blocking innoncent things (like what happened in my biology lesson, although the security settings were stupidly tight on those computers) and still allowing a lot of pornography to be viewed.
Like someone on here mentioned, won't it end up blocking results where people are researching statistics/news reports of child abuse/pornography !?!?