The BBC is reporting the following message which they found on an Islamic website. I'm not sure if it's just an opportunistic group claiming credit for the attacks or if it's real, either way it's hard to comprehend the mindset of the writers.
In the name of God, the merciful, the compassionate, may peace be upon the cheerful one and undaunted fighter, Prophet Muhammad, God's peace be upon him.
Nation of Islam and Arab nation: Rejoice for it is time to take revenge against the British Zionist Crusader government in retaliation for the massacres Britain is committing in Iraq and Afghanistan. The heroic mujahideen have carried out a blessed raid in London. Britain is now burning with fear, terror and panic in its northern, southern, eastern, and western quarters.
We have repeatedly warned the British Government and people. We have fulfilled our promise and carried out our blessed military raid in Britain after our mujahideen exerted strenuous efforts over a long period of time to ensure the success of the raid.
We continue to warn the governments of Denmark and Italy and all the Crusader governments that they will be punished in the same way if they do not withdraw their troops from Iraq and Afghanistan. He who warns is excused.
God says: "You who believe: If ye will aid (the cause of) Allah, He will aid you, and plant your feet firmly."