In the UK, people have been convicted and/or found liable on the basis of conspiracy to defraud and/or encouraging or facilitating criminal copyright infringement.
At trial, one person has been convicted on the basis of conspiracy to defraud. There is no "facilitating copyright infringement" law per se. That case was a private prosecution and is pending appeal (something like 20-30 different grounds), which will hopefully be heard before FACT Ltd can do too much bullying on the basis of it (it was a very dodgy conviction; the judge instructed the jury that what he was doing was illegal because what he was doing was illegal, basically). The only other criminal cases that have gone to trial resulted in acquittals or dismissal of charges.
As for civil cases, there's really only the Newzbin 1 case, which found a company running a website was liable for copyright infringement mainly due to the extra steps they took to help their premium members. In any case, "contributory copyright infringement" isn't the right term; you're looking at either direct (or secondary, but that usually involves businesses) copyright infringement, or joint liability for someone else's infringement. There is no blanket "you were somehow involved in someone else's infringement therefore you must be liable!!" thing.
The s107(2A) offence is an interesting one - to my knowledge it has never gone to trial. While there have been a couple of summary convictions under it, they don't really count (and, to be blunt, neither do un-appealed cases at trial). It's only been in the last couple of years that the police (or rather, FACT Ltd / the BPI) have realised this is the correct offence, and before that managed to get some summary convictions under s107(1)(e) - despite it not applying.
However that is not their main purpose, and there is no reason why the courts have to treat them the same way, any more then convicting a rioter for handing out bricks at a riot means they have to convict every building site foreman in the country.
But that's not quite the way the law works; the key thing here is the "mere conduit" (and similar) principles found in the e-commerce regulations (regs 17-19). This one provides an absolute defence to a damages claim for anything done through a mere service provider - such as someone hosting a proxy.
What matters is not whether they transmit copyright information, but whether that transmittion is authorised. If my ISP helps me view the NYTimes website, I am view copyright material with authorisation.
As an aside, this isn't how the law works either. Currently you would need a licence to view the content on the NYTimes website. If you didn't have a licence, you would be liable for copyright infringement (as would oyur ISP, but for the above limitation). Fortunately, the NYTimes website includes a term saying that "You may download or copy the Content and other downloadable items displayed on the Services for personal use only, provided that you maintain all copyright and other notices contained therein." However, go beyond that - such as by viewing it at work - (and not within the scope of a statutory limitation to copyright) and you're breaking the law. There's case law on that (which, hopefully, the Supreme Court will be overturning in the next couple of months). Interestingly, this also means that were I (or someone else) to email you some content from the NYTimes website while at work, *you* would be liable for copyright infringement as soon as you download the email to read it.
Of course, if the website doesn't have an explicit licence, you have to rely on implied licences. But that can be overridden by an express one -so if you're in the UK (or England or Wales, at least) and see the phrase "All Rights Reserved" while browsing the Internet, there's a good chance you've broken the law.
Isn't copyright law fun?
Anyways, my understanding of the situation is that PPUK *isn't* breaking the law because it is not actually infringing any copyrights, nor is it authorising such an infringement, nor is within the scope of joint tortfeasance (due to acting with a common purpose to infringe copyrights). Additionally, it is covered by the "mere conduit" limitation, so at most copyright owners could get would be an injunction (but that's an equitable remedy, so there are issues with the substantial delay, a lack of proportionality, and acting in vain). If the case did go to trial, it would be rather interesting.
But it won't, because the 6 guys in question don't have the few hundred thousand pounds they need lying around. At the moment, the proxy is down. But that's the current British justice system for you - only works if you have money.
[Disclaimer: I am a member of, and work for, the Party. I am not one of the 6 members in question and so not directly involved. I am not a lawyer, but have studied copyright law in some detail.]