Saudi national Shaker Aamer was beginning his first full day back in Britain on Saturday after flying back to London from the prison on Cuba.
The 46-year-old father of four thanked his supporters, “so strongly devoted to the truth,” for helping him through the ordeal.
“If I was the fire to be lit to tell the truth, it was the people who protected the fire from the wind,” he said in a statement issued through his lawyers.
“Without knowing of their fight I might have given up more than once… and without their devotion to justice I would not be here in Britain now.”
British newspapers reported he plans to claim for damages from the British government.
Members of his legal team told The Guardian newspaper they expect the government to settle the claim as quickly as possible to avoid allegations of Britain’s involvement in human rights abuses aired in court.
The nine British nationals and six British residents released from Guantanamo are each thought to have received around £1 million ($1.55 million, 1.4 million euros) from the government.
London settled civil damages claims rather than contest in court allegations that Britain’s security services were complicit in what happened to them.
Lawyer Clive Stafford Smith, who spent years campaigning for Aamer’s release, told AFP he would now be given medical tests before being reunited with his family.
He is thought to be suffering from a string of health issues, including post-traumatic stress disorder.
“I think he’ll want to hear the word ‘Daddy’ instead of number 239,” said Smith, referring to Aamer’s prisoner number at Guantanamo.
The United States had accused Aamer of acting as a recruiter, financier and fighter for Al-Qaeda, as well as being a close associate of Osama bin Laden, but never charged him.
Twice cleared for release from Guantanamo in 2007 and 2009, he has always denied the allegations and said he was in Afghanistan working for a charity when he was captured by bounty hunters.
Cameron had pressed US President Barack Obama for his release and his spokeswoman said there were no plans for the British authorities to detain him or charge him.
The Daily Telegraph newspaper said it understood that high-level discussions had taken place between London and Washington on how Britain’s security services would “monitor potential threats” posed by Aamer.
Aamer was born in Saudi Arabia in December 1968 and lived in the US before settling in Britain, where he married a British woman and, in 1996, became a resident.
In 2001, he took his family to Afghanistan, but sent them to Pakistan after the September 11 attacks. He said he was about to join them when he was detained.
He was transferred to Guantanamo Bay in 2002, where he said he faced mistreatment, leading him to become an advocate for prisoners’ rights and an organiser of hunger strikes.
Aamer fasted even after his release was announced last month in protest in protest at alleged mistreatment.
Obama signed an order six years ago to close the facility, which was opened to hold terror suspects following the September 11, 2001 attacks on the United States.
But he has struggled to do so in the face of opposition in Congress and with other countries reluctant to take in one-time terror suspects.
Following the release of Mauritanian detainee Ahmed Ould Abdel Aziz from Guantanamo this week, the US defence ministry said there were 113 detainees left.