Ray Saintonge said:
Many Wikipedians do not edit under their own names. What standards of evidence do the British courts follow when the defence is simply, "It wasn't I who said that." The burden of proof lies with the plaintiff, and someone needs to accept responsibility for the costs to a defendant who has been misidentified.
The burden of proof lies with the plaintiff in establishing that the defendant made the edits, but this is decided on the balance of probabilities. A reasonably resourceful plaintiff could track down most Wikipedia editors easily enough. But the neat thing about the UK system, from the plaintiff's point of view, is that he can sue the website. If he notifies the website of the problem and they don't take it down fairly promptly, they're wide open. Not doing this in 1997 (in that case, in relation to a Usenet post that wasn't even made on their servers) cost Demon Internet a lawsuit, which eventually led to about USD 1,000,000 settlement, plus whatever costs they incurred in a lawsuit that dragged on for over a year. FOr the purposes of defamation in UK law, a website is a publisher.
The case in US law is very different because of the CDA 230(c)(1).