I don't think the McLibel applies very much to restaurant reviews. In the McLibel case, the defendants defamatory and unsubstantiated writings were presented as fact, not opinion. The judge specifically called that out as the crux of why it constituted libel. I think that pamphlet would have been actionable in the USA, too.
A restaurant review, on the other hand, is presented as personal opinion (that is what "review" means), and the average reasonable reader would recognize it as such.
Even in light of McLibel, I think that as long as the reviewer gets his or her facts straight, the opinion part of the piece is not actionable, however negative it might be. An example:
Saying, "the steak arrived with maggots crawling all over it" when in fact it wasn't presented this way would be actionable.
Saying, "their steak was overdone and tasted like a piece of shoe leather" is not actionable, as it is expressing a personal opinion.
-Paul W.