The defendants pleaded not guilty to the charges and are appealing their convictions.

Melanie Evans, a charity worker, was one of the six people who cut the fence and helped carry blockade materials to the side of the parked Boeing 767 jet. She said that the group’s view was that Britain was compromising the safety of the migrants who were to board the flight.

“At the time of our action, it seemed that there was no just way to protect these people because they were so quickly and secretly snatched away,” Ms. Evans said in a phone interview on Wednesday.

Ms. Evans said she knew what she was doing was against the law, but she said she had expected a charge of aggravated trespassing, which has been used in previous cases.

Raj Chada, who represented the defendants in court, said in a statement that the charges amounted to using “counterterrorism legislation.”

“While we are relieved that none of our clients face a custodial sentence, today is still a sad day for justice,” he said, adding that the group “will continue to fight in the appeal courts to get these wrongful convictions overturned.”

Protesters in Britain and other countries have staged similar protests to try to halt deportation flights. In July, a woman in Sweden refused to sit down in a Turkish Airlines flight, trying to block the removal of an Afghan man from the country. The woman, Elin Ersson, live-streamed the video, drawing praise from around the world, but she faces a fine or a prison sentence of up to six months if convicted of violating a Swedish aviation law.

Last year, Prime Minister Theresa May apologized after some members of the “Windrush generation,” who came to Britain legally from colonies or former colonies from 1948 to 1973, were threatened with deportation because they could not prove they were residing in the country legally.