Binyam Mohamed landed at Northolt military base Monday - the first Guantanamo prisoner released since President Barack Obama took office.
British authorities said Mohamaed would be briefly interviewed by police and immigration officials before being released. He has to apply for temporary residency since his status expired during his detention.
"I hope you will understand that after everything I've been through, I am neither physically nor mentally capable of facing the media on the moment on my arrival back to Britain," Mohamed said in a statement released through his attorneys before his plane landed.
Mohamed's case is raising uncomfortable questions for Obama - who has promised a new era of government accountability - and for Britain, America's closest partner during its war on terror.
Lawyers on both sides of the Atlantic are suing for secret documents they say prove the United States sent Mohamed to Morocco where he was tortured and prove that Britain knew of the mistreatment - a violation under the 1994 U.N. Convention Against Torture.
Britain's Attorney General has opened an investigation into whether there was criminal wrongdoing on the part of Britain or a British security agent from MI5 who interrogated Mohamed in Pakistan, where he was arrested in 2002.
Two senior British judges, meanwhile, have reopened a case into whether 42 secret U.S. intelligence documents shared with Britain should be made public. The judges say they ruled to keep documents - which detail Mohamed's treatment - secret last month because of a British claim it could hamper U.S. intelligence sharing.
Several other lawsuits are underway in the United States against a Boeing subsidiary that allegedly supplied planes for rendition flights to Morocco and for the disclosure of Bush-era legal memos on renditions and interrogation tactics.
"I am so glad and so happy, more than words can express," said Mohamed's sister, Zuhra Mohamed.
The 30-year-old Ethiopian refugee has few remaining links to Britain. His brother and sister live in the United States. His parents are said to be back in Ethiopia. And his British residency that he obtained when he was teenager has since expired.
Any revelations from the lawsuits could be particularly damaging for the British government, which unlike the Obama administration, doesn't have its predecessors to blame. Prime Minister Gordon Brown's Labour Party has been in power for more than a decade.
"I assure you that we have done everything by the law," Brown told reporters last week when faced with questions over Mohamed's case.
Mohamed's family came to London from Ethiopia in 1994. They applied for asylum following the ouster of Marxist dictator Mengistu Haile Mariam's ouster but they were only given temporary residency. Mohamed's residency was to be renewed in 2004, the year he arrived in Guantanamo.
Schooled in West London, Mohamed worked as a janitor and later became a student of electrical engineering before converting to Islam in 2001.
Shortly afterward, he said he went to Pakistan and Afghanistan to escape a bad circle of London friends and experience an Islamic society. But he was detained in the Pakistani port city of Karachi in 2002 for using a false passport to return to Britain.
For three months, he says he was tortured by Pakistani agents, who hung him for a week by a leather strap around his wrists. He says at least one MI5 officer questioned him there.
He claims he was handed over to U.S. authorities in July 2002, and then sent to Morocco where he was tortured for 18 months. According to his account, one of his foreign interrogators slashed his penis with a scalpel.
Many of the estimated 750 detainees who have passed through Guantanamo prison camp since it opened in January 2002 have reported mental and physical abuse, but few have detailed such sustained physical and mental abuse at an alleged CIA covert site.
Mohamed claims he eventually confessed to an array of charges to stop his abuse - a confession that laid the groundwork for his transfer to another CIA site in Afghanistan, where he said he was starved and beaten before being sent to Guantanamo in 2004.
The United States refuses to account for Mohamed's whereabouts for 18 months but has previously denied sending terror suspects to countries with track records of torture. British authorities, such as former Foreign Secretary Jack Straw, have said they depended on those U.S. assurances.
Mohamed will be met by a doctor and his lawyers, Clive Stafford Smith and Gareth Pierce.
"He is a victim who has suffered more than any human being should ever suffer," Stafford-Smith said.
In May of 2008, Mohamed was charged with conspiring with al Qaeda members to murder and commit terrorism. He was also accused in a "dirty bomb" plot to fill U.S. apartments with natural gas and blow them up.
But then in October all charges were dropped - only months after his lawyers filed a lawsuit in Britain for the disclosure of the 42 secret documents.
Two other former British residents remain in Guantanamo: Saudi-born Shaker Aamer, 37, and Algerian Ahmed Belbacha, 39.
By Associated Press Writer Paisley Dodds