While they were given permission to meet their lawyers and a family member, Scotland Yard said they would be given a medical examination to assess their mental and physical states.
The men were flown to England aboard a Royal Air Force aircraft after spending three years in U.S. custody.
Under Britain's Terrorism Act, the men can be held for 14 days without being charged. Meanwhile, they will be held in a high security London prison for interrogation.
"We have an absolute duty on behalf of all communities to investigate the circumstances leading to the men's detention," said Scotland Yard's Deputy Assistant Commissioner Peter Clarke. "We have discussed this case with members of the Muslim community and recognize that there are strong feelings about the return of these men to (Britain)."
The men's lawyers are reportedly considering launching compensation claims against the United States, for alleging the men were unlawful combatants in the Afghanistan war.