A separate new outbreak of the disease was detected in Democratic Republic of Congo.
A specially adapted Royal Air Force cargo plane picked up the male healthcare worker in Sierra Leone on Sunday after British Foreign Secretary Philip Hammond authorised his repatriation for treatment.
The Department of Health said the patient - whose identity has not been disclosed - was "not currently seriously unwell". The man will be transported to an isolation unit at the Royal Free Hospital in London.
The hemorrhagic fever has killed at least 1,427 people, mostly in Sierra Leone, Liberia and neighbouring Guinea, the deadliest outbreak of the disease to date. The disease also has a toehold in Nigeria, where it has killed five people.
In Democratic Republic of Congo, Health Minister Felix Kabange Numbi said an Ebola outbreak had been confirmed in the remote northern Equateur province - 1,200 km (750 miles) from the capital Kinshasa - but it was a different strain of the virus from the West African one.
There have been six outbreaks of Ebola in Democratic Republic of Congo since the disease was discovered there in 1976, with a total of more than 760 deaths.
The World Health Organization (WHO) has said that more than 225 health workers have fallen ill and nearly 130 have lost their lives to Ebola since the West African outbreak was detected in the jungles of southeast Guinea in March.
The Boeing C-17 plane carrying the British medic left the Sierra Leonean capital Freetown at around 1250 GMT. Television footage showed the plane arriving at RAF Northolt air base in Britain after a flight of just over 7 hours. An ambulance backed up to the plane to receive the patient.
Britain's Deputy Chief Medical Officer John Watson said final approval for the evacuation was given by a team of physicians who had travelled to Freetown on the plane.
"We understand that this patient, during the course of the work that he was carrying out, was exposed about a week ago and became unwell two or three days ago," Watson told Sky News.
The Royal Free Hospital has Britain's only high-level isolation unit for treatment of infectious diseases, as well as a team of specially trained staff.
Paul Cosford, director for health protection at state body Public Health England, said strict protective measures were being taken to minimise the risk of transmission when transporting and treating the individual in Britain.
Two U.S. doctors, who contracted Ebola in Liberia and were evacuated to the United States, were discharged from hospital last week after receiving treatment with an experimental drug, ZMapp. It was not clear what role it played in their recovery.
Its U.S.-based manufacturer, Mapp Biopharmaceutical, has said limited supplies of the drug have already been exhausted after it was used to treat three African doctors in Liberia.
FIRST WHO WORKER TAKEN ILL
In Democratic Republic of Congo, in central Africa, dozens of people have died in Equateur from a mysterious disease in recent weeks, but the WHO said on Thursday it was believed to be a kind of hemorrhagic gastroentiris.
However, Minister Kabange Numbi said that two out of eight cases tested had come back positive for Ebola and he said the region around the town of Djera would be quarantined. He said 13 people had died from the disease, five of them healthcare workers, and some 80 others were being monitored.
"This epidemic has absolutely no link with what is happening in West Africa," the minister told a news conference.
Numbi said that one of the two cases that tested positive was for the Sudanese strain of the disease, while the other was a mixture between the Sudanese and the Zaire strain - the most lethal variety. The outbreak in West Africa is the Zaire strain.
A WHO spokeman said it could not confirm the results of the tests, which were conducted by Congolese authorities.
Separately, the WHO said a healthcare worker it had deployed to Sierra Leone had tested positive for Ebola. It was the first time someone working under the aegis of the WHO has been infected, it said in a statement.
A government source in Sierra Leone, who asked not to be identified, said the worker was a Senegalese expert deployed by the WHO in the eastern town of Kailahun.
The WHO has sent in nearly 400 people from its own staff and partner organisations since the outbreak was detected in March.
It is the first outbreak of the disease in West Africa and the worst since it was discovered in 1976 in the jungles of Democratic Republic of Congo, then known as Zaire.
The WHO is due to release next week a draft strategy to combat the disease in West Africa. The U.N. agency has faced criticism that it moved too slowly to contain the outbreak.
With the healthcare systems of Sierra Leone and Liberia already fragile following a decade of civil war in the 1990s, and still lacking staff, the WHO said a surge in foreign healthcare workers was essential.
The WHO has warned that a decision by many transport companies to suspend services to Ebola-hit countries was leading to shortages of basic goods and foodstuffs. The U.S. Agency for International Development (USAID) airlifted more than 16 tons of medical equipment and emergency supplies to the Liberian capital Monrovia on Sunday.