Confirming the release of the detainees, Britain's interior ministry said the three men's return to Britain did not mean they would be permitted to stay in the country.
"[This] does not imply commitment on our part that they can remain permanently in the United Kingdom. Their immigration status will now be reviewed," a spokeswoman said.
But Katnelson, who is also a representative of Reprieve, a human-rights group, said: "I think if the British government really thought they were a danger, they wouldn't be bringing them back."
The US has said that the men pose a danger but the British government has not said what security measures it may impose on them when in the country.
Katznelson said the men had voluntarily agreed to security arrangements demanded by the British authorities, but declined to give details.
He said el-Banna, a father of five, had "serious health issues" with untreated diabetes.
El-Banna was arrested by Gambian authorities with Bisher al-Rawi, an Iraqi national, in November 2002 and transferred to US detention.
Al-Rawi was released from Guantanamo in April after five years in detention. It emerged he had assisted MI5, Britain's domestic spy agency.
Amnesty International said Deghayes was captured in Pakistan in 2002, along with Shaker Aamer, a Saudi national.
The British government changed its policy of refuse to seek the release of non-British nationals in August.
The US agreed to free el-Banna, Deghayes and Sameur earlier this month but refused to release Aamer and Binyam Mohamed, an Ethiopian national.
David Miliband, foreign minister, said last week that those men will remain in Guantanamo.
The US has been criticised internationally for its detention policies at the prison camp.
It has said in the past that it does not want the facility to remain open for a protracted period of time.