The Light Pacifics, West Country Class 7P5F and Battle of Britain Class 7P5F, were designed by O.V.S Bulleid as a lighter version of the Merchant Navy Class. Features included air-smoothed casing and Bulleid's innovative, if problematic, chain-driven valve gear. The concept was to save weight while retaining maximum power, creating a fast and powerful mixed traffic engine, allowing the Light Pacifics to work almost all of the Southern Region.
Initially introduced in 1945, a total of 110 Light Pacifics were built out of the Brighton and Eastleigh Locomotive Works. 66 early members of the Class were named after cities, towns and tourist spots in the West Country which were served by the Southern Railway. In 1946 the Class began working the Central and Eastern Sections and as a result the remaining 44 locomotives were named after aircraft, airfields, Royal Air Force squadrons and key personalities of the Battle of Britain. Other than their nameplates, the West Country and Battle of Britain locomotives were completely identical.
Construction continued after Nationalisation in 1948 until 1951. 19 of the Class were built after Nationalisation with large 5,500 gallon capacity tenders. However, the original Southern Railway number series, based on Bullied's typically individualistic European 21C100 style, was superseded and the colour scheme updated to standard British Railways express passenger loco Brunswick Green, lined in black and orange.
Despite the power of Bulleid's boiler, by the 1950s the failings of these Light Pacifics were beginning to show. The demanding level of maintenance, high fuel consumption and numerous problems caused by the extremely high power to weight ratio, forced a major overhaul.
Between 1957 and 1961 a total of 43 West Country and 17 Battle of Britain locomotives were rebuilt into more conventional engines. There were many alterations and features adopted from the BR Standard locomotives that had been introduced since Nationalisation, with fundamental changes including replacement of the chain gear with Walschaerts valve gear and the removal of the air-smoothed casing, which was replaced by large square smoke deflectors, creating a slightly slower, heavier but far more reliable engine. Electrification inevitably spelt the end for the Class with only 21 remaining until the last day of Southern steam on the 9th July 1967.
Twenty of the Class survived and are in various stages of restoration and preservation with heritage railways. Five of the Class 34007 'Wadebridge', 34028 'Eddystone', 34046 'Braunton', 34067 'Tangmere' and 34070 'Manston' are certificated and operational.