Then why were they freed? Because of increasing political pressure on Guantanamo. From the [1] Times Online, :

The four Britons sent home from Guantanamo Bay on Tuesday and set free from police custody last night still pose a terrorist threat, the United States said. Lieutenant Commander Flex Plexico, a Pentagon spokesman, said: "We continue to believe that these individuals pose a significant threat. British authorities have assured us that they will take the necessary steps to address the threat posed by these individuals."