Then why were they freed? Because of increasing political pressure on Guantanamo. From the 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.”