This is a British law to stop British people looking at things the Tories think nobody but themselves should be allowed to view.
How will it be enforced on foreign sites? Will they order all UK ISPs to block an ever-increasing list of foreign sites? If they can't block the Pirate Bay how are they going to block every single one of these?
How will a site know that the visitor is currently in Britain? If they are just using IP address there are all sorts of ways round it. What happens when they block someone who is in another country from viewing entirely legal sites?
What about a mixed content site? Are they going to block every Google blog because some have adult content? The whole Wikipedia site once got blocked due to a single image.
The USA has some very strict laws on drinking age - getting a Fake ID is practically a rite of passage!
This is going to be seething with false negatives and false positives.