EU not UK...
It is illegal under EU data protection´and has been implemented as such in Germany. Here. the Baden Württemberg state data protection commission were up in arms about the way Facebook and Google et al can trace users, even those who don't use their services, by tracking IP addresses and cookies using JavaScript embedded in 3rd party websites.
heise.de developed a JQuery module ( http://www.heise.de/extras/socialshareprivacy/ ) to get around the problem, by disabling the buttons by default and only enabling them, once the user "slides" them to the enabled position.
This was back in August, when the story first surfaced.