U.S. President Barack Obama is greeted by U.S. Ambassador to Britain, Matthew Barzun (L) and his wife Brooke, after arriving at Stansted Airport near London, Britain April 21, 2016. REUTERS/Peter Nicholls 

U.S. President Barack Obama walks down the steps of Air Force One as he arrives at Stansted Airport near London, Britain April 21, 2016. REUTERS/Peter Nicholls 

U.S. Secretary of State John Kerry leaves Stansted Airport to return to the United States from London on May 16, 2014. REUTERS/Jacquelyn Martin/Pool (BRITAIN - Tags: POLITICS) 

Passengers queue in Terminal 3 at Heathrow Airport in west London December 7, 2013. Thousands of travellers were delayed at major airports across England on Saturday because of a technical problem at an air traffic control centre. Thousands of travellers were delayed at major airports across England on Saturday because of a technical problem at an air traffic control centre. Disruption was reported at most airports including Heathrow, Gatwick and Stansted, after what National Air Traffic Services (NATS) said was a computer glitch at its control centre in Swanwick in Hampshire. REUTERS/Luke MacGregor (BRITAIN - Tags: TRANSPORT BUSINESS TRAVEL) 

U.S. President Barack Obama and first lady Michelle Obama step off Air Force One as they arrive at Stansted Airport near London, March 31, 2009. Obama arrived in Britain on Tuesday for this week's G20 economic crisis summit, starting his first major trip abroad since taking office on Jan. 20. REUTERS/Jim Young (BRITAIN POLITICS) 

A passenger waits for delayed flights at London Stansted airport in Essex February 8, 2007. Travel chaos ensued on Thursday as much of the country was covered by a thick blanket of snow at the height of the morning rush-hour. REUTERS/Alessia Pierdomenico