@Ex0 - you can't get subpoenas in this country, so it looks like you may have been looking at American procedures.
The Secretary of State (and anyone representing him) has a right to obtain details from all UK ISP's and doesn't need a warrant. If they fail to provide the details on request (or fail to keep satisfactory records), they can be prosecuted.
Most anonymizer sites are in the game of making money, they are much more cooperative than they would like users to believe, they don't want to lose their revenue by being shut down but they also don't want the users to know they cooperate either, so a cozy arrangement generally exist.
There are other methods of tracking a changing IP which are automated, only trips through VPN are immune to tracking, but outside of the VPN they can still be tracked.
Its quite easy - the internet is bi-directional, you can't have a situation where you put a page request out on one IP and it arrives back on another without it being obvious that you pass an instruction for the expected return IP, otherwise it wouldn't know where to go.
If you use Modem access to a foreign ISP then it may start to get difficult, but not many would bother to get stuck with slow modem speeds again.
@nobody - its you that just raised the topic that you say shouldn't be brought out at every opportunity