The A.27 London was designed in response to the Air Ministry Specification R.24/31 issued for a "General Purpose Open Sea Patrol Flying Boat" and was based on the Saro A.7 Severn. The London and its contemporary, the Supermarine Stranraer, were the last multi-engine, biplane flying-boats to see service with the RAF. The design utilized an all-metal corrugated hull and fabric-covered wing and tail surfaces, with two Bristol Pegasus II radial engines, mounted on the upper wing to keep them clear of spray while taking off and landing.
The first prototype flew in 1934 and then went on to serve until 1936 with 209 and 210 Squadrons RAF at RAF Felixstowe and Gibraltar respectively.
The first deliveries of production aircraft began in March 1936 with Pegasus III engines, and from the eleventh aircraft onwards the Pegasus X engine was fitted instead and the aircraft's designation changed to London Mk.II. Earlier Londons were retrofitted with the Pegasus X and were also given the "Mk.II" designation. This model equipped 201 Squadron RAF at RAF Calshot, replacing Supermarine Southamptons. Others were delivered in October of the same year to 204 Squadron RAF at RAF Mount Batten, Plymouth, also replacing Southamptons. More were delivered to the same squadron the following year to replace Blackburn Perths, and to 202 Squadron RAF at Kalafrana, Malta, replacing Supermarine Scapas, and 228 Squadron RAF at Pembroke Dock.
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. In this configuration they had a range of 2,600 miles (4,180 km).
- Prototype : One only
- London Mk.I : 10 built with two 820 hp Pegasus III engines and two-bladed propellers. Later converted to Mk II.
- London Mk.II : 20 built with two 915 hp Pegasus X engines and four-bladed propellers.