Although due for replacement by the outbreak of World War II, they saw some active service pending the introduction of the ultimately unsuccessful Lerwick.

The A.27 London was designed in response to the Air Ministry Specification R.24/31[1][2] issued for a "General Purpose Open Sea Patrol Flying Boat" and was based on the Saro A.7 Severn.

In 1937–1938 204 Squadron RAF used five Londons equipped with auxiliary external fuel tanks for a long-distance training flight to Australia to celebrate the 150th anniversary of the founding of the Colony at Sydney, New South Wales.

Armament in the form of bombs, depth charges, and naval mines up to a total weight of 2,000 lb (907 kg) could be carried beneath the lower wings.

Gradually, the Londons' duties were assumed by newer aircraft such as the Lockheed Hudson, while squadrons flying Atlantic and Mediterranean patrols were re-equipped with Short Sunderlands.