Col Nick Handy, the senior explosives officer leading the operation, said the bomb could not be moved because its fuses could not be fully assessed.
He said X-ray examinations had failed to give a clear picture of one of the fuses, which meant it had to be blown in situ.
Drones were seen hovering over the site throughout the morning and a bomb disposal robot was sent to Southwark Drive.
Handy said the blast was caused by about a third of the explosives in the bomb, which was about 40mm (1.5in) thick.
“It could have been a lot bigger,” he said during a press conference.
“We set out yesterday to deflagrate that item in order to limit the blast affects and we achieved that successfully.”
He added the bomb burned at about 1,000C (1,832F) for about 10 minutes.
“The guys and girls on the ground were excellent in everything that they did – we won,” Handy said.