Servicemen at the Royal Air Force Waddington in Lincolnshire manned the Reaper drone 2,000 miles away in Abu Kamal in eastern Syria on May 9.
In the footage shown Wednesday, Islamic State militants can be seen leading two handcuffed prisoners from a van and marching them in front of a large crowd of civilians. The drone then released a Hellfire missile that hit an Islamic State sniper on a nearby roof. The fighters fled before they could perform the execution.
"The individual whom we engaged was a sniper in over-watch to shoot civilians who sought to move away from the execution, let alone to protect the planned execution itself," Air Commodore Johnny Stringer, commander of British air operations in Iraq and Syria, said. "That particular example for us very much brought it home because civilians had been herded in, forced literally at gunpoint, to go and watch this going on in their hundreds."
The Ministry of Defense said it couldn't attack the fighters with ground forces because it would have put the civilians at risk.
"By being members of [the Islamic State] and frankly engaging the people we are here to protect, they become valid military targets and that's the way we look at it," Stringer said. "These people know we can find them wherever they try to hide."
The Syrian Observatory for Human Rights said the strike may have injured up to 20 civilians and killed multiple militants.
Officials said the military drone pilots involved in the May strike could be awarded medals.
"The changing character of warfare provides new challenges; not just about how we fight but also how we recognize and support those who serve," British Defense Secretary Michael Fallon said during a visit to British troops in Iraq. "As fighting has evolved, we have adapted, ensuring our troops have cutting-edge equipment including unmanned systems operated from outside the battle space. Our recognition of service, the risks taken, and the longterm effects must therefore adapt too."
Britain has more than 1,400 service personnel as part of the coalition fighting the Islamic State, including 600 soldiers on the ground in Iraq.