He added he was "hugely grateful" to Teodora Obiang Nguema, Equatorial Guinea's president, for pardoning him.
"Now I am relieved that the coup d'etat we tried to lead in 2004 was not successful," he was quoted as saying.
The west African state also granted amnesty to four other mercenaries, including Nick Du Toit, Mann's South African's co-conspirator.
Mann was arrested in March 2004 along with 61 other suspected coup plotters when their aircraft landed in Zimbabwe.
The group had planned to overthrow Teodora Obiang Nguema, who has ruled the country since 1979.
Mann and Du Toit had set up Executive Outcomes, which operated from Pretoria in South Africa and helped the Angolan government to protect its oil installations from rebels during that country's civil war.
Mann's arrival in Britain comes a day before Equatorial Guinea opens its presidential election campaign.
The vote is set for November 29 and Obiang Nguema is seeking another mandate.
The former Spanish colony is Africa's third biggest oil producer after Nigeria and Angola following the discovery of large offshore oil deposits in the early 1990s, but the benefits have yet to trickle down to the people.