There's no "possibly" about it, this would be trivial with IPv6. It wouldn't even require international agreement, since a single government could acquire a range of a google or so addresses and pass a law making it illegal (in their country) to put a system on the internet with an IPv6 address outside that range. It wouldn't affect end-users at all, since they all get their addresses dynamically from ISPs, and the ISPs would comply, joyfully, since this is *much* easier than anything else that governments have threatened to make them do.
As for why, read the article. Politicians and Joe Public alike are constantly bemused by why something can't be done about <whatever> on the web, so perhaps it is time the IT people took that as a serious and legitimate feature request. Then read the innumerable articles about malware, which all end up talking to controller in countries that I and most others (including probably your good self) could quite easily live without a direct net connection to. Lastly consider email spam. 99% of all legimate email to me comes from my own country. I know *my* spam filter would love to use that fact, but under the present chaos I have no way of telling it.
As for proxies, VPN tunnels and similar, if they result in IP packets with a source address that I'm blocking (not my ISP, unless I've told them to, because this is my decision) my kids still won't be able to connect. Conversely, if they claim to be sourced in this country, this would count as re-publishing the material in the UK and that would open up whoever ran the proxy to whatever laws apply here. (Of course, a multi-national company that simply wanted a point of presence, rather than a data centre, in every country might very well decide that this was what they wanted.)
Wait until that suggestion is run past the politicians. They'll have the necessary law on the statute books by tomorrow. It is just what they've been looking for: a way of forcing IP packets to advertise electronically whether they agree to be bound by local law. On top of that, people can build whatever filtering rules they want and politicians can claim the credit for cleaning up the internet.
Yes, there will still be porn on the web, but next time the children find it there, it will be for one of three reasons:
i) negligent parent failed to block foreign sites (so you can't blame the politicians),
ii) negligent parent failed to implement filtering based on parental controls (that are now mandatory for all local sites, so you can't blame the politicians),
iii) someone broke local laws (so send in the local police and don't blame the politicians)
Yes, too, the system could be abused by governments that want to dictate filtering at a national level, but that's a problem with such governments, not my proposal. This is just a filtering tool that could, with modest legal support, be made reliable. It's up to you what you let your government do with it.