The first member of the class was weighed at Brighton Works on VE day, was found to be within the estimated weight and emerged in June 1945 resplendent in pre-war Malachite livery - in sharp contrast to the prevailing starkness of the wartime overall black livery. The somewhat unfortunate class name of "Lightweight Pacific" was replaced by "West Country" with, subsequently, engines built for the south-east (though, as it turned out, not necessarily used there) named as "Battle of Britain". The initial order was for a bold 70 locomotives and the build was subsequently extended through and after nationalisation to a total of 110.

Recognising the Southern's close association with the west of England, and also that this class of locomotive would be able to serve the area comprehensively, some 66 were named after places and features in that area whilst some 44 were named after squadrons and other aspects associated with the Battle of Britain. These were referred to as the Battle of Britain "class" but it must be remembered that in all aspects other than the naming they were identical locos to the WC "class". Although the engines were prone to similar problems encountered with the MN class the Southern stole a march on the other railway companies and gained a considerable amount of public support by at the time introducing a bold forward looking design wearing a bright livery and by their naming policy honouring the war effort. Far, far from Waterloo, beyond Okehampton to Launceston and Padstow, from Barnstaple via the 1 in 36 Morthoe bank to Ilfracombe, over Meldon viaduct or along the GWR route to Plymouth, on the S&D line to Bath, the "West Country" pacifics were doing their stuff, and more than holding their own if rostered for a MN turn to Salisbury.

Engines were built as follows:
21C101-21C148 (renumbered 34001-34048) 1945/46 WC "class" (narrow cab)
21C149-21C170 (renumbered 34049-34070) 1946/47 BB "class" (narrow cab)
34071-34090 1948/49 BB "class" (wide cab)
34091-34108 1949/50 WC "class" (wide cab)
34109-34110 1950/51 BB "class" (wide cab)
all were built at Brighton works except 34095/7/9/101/2/4 which were built at Eastleigh

"Class" is in quotes as there was no difference between the Battle of Britain and West Country locomotives, other than the style of their nameplates. All of the narrow cab locos were originally supplied with a 4500 gallon capacity tenders but the 1948/49 BB batch came with the heavier 5500 gallon tenders. From 1952 onwards BR modified all bar five of the tenders by removing the raves to allow easier coaling and access for the water column bag when filling the tank. (Note that the modifying of tenders preceded by up to 5 years the modifying of many of the locomotives.) Corrosion, buckling and split welds were a big factor in the need to modified the first 70 as the metal was too thin, stemming from efforts to cut down on weight, and areas such as the joints of the raves which allowed water to collect. Between 1954 and 1956 the boiler pressure was lowered to 250 psi.

As with the Merchant Navy class despite their failings these locomotives were capable of impressive performances due in part to the steaming capacity of the boiler which was capable of a sustained high output. Their wide route availability meant they were useful engines that could traverse most of the system. Three WC locos took part in the 1948 locomotive exchanges and being driven by expert crews put up some outstanding performances albeit with a high coal consumption. Two WC class engines were experimentally converted to oil burning but were rapidly converted back to coal burning when the experiment was cancelled.

When in 1955 authority was given for modifyding half of the MN Pacifics it was also given for 15 light Pacifics. However unlike the MN class not all were modified since the financial justification for converting a relatively new locomotive class was weak especially with the end of steam already in sight. In all 60 out of 110 were modified with the WC class being modified in greater proportion than the BB class - 43 out of 66 WC locos compared with 17 out of 44 BB. For information and pictures of modified light pacifics please follow the link below to the modified light pacific pages. So a good quantity of air-smoothed examples did survive working useful lives until the end of steam and working on the "Withered Arm" in particular since the heavier weight of the modified excluded them from working north of Meldon Junction to Bude and Padstow or to North Devon.

34064 Fighter Command was one of two locomotives on British Railways to be fitted with Dr Giesl's Oblong Ejector. The Lemaître five nozzle blastpipe as fitted to the light pacifics did not work that efficiently with a spark arrester, so the seven nozzel Giesl spark arresting arrangement was tried in August 1962, and proved to be a great success. The increased exhaust velocity of the Giesl ejector was found to greatly improve smoke lifting and a leaflet to BR drivers at the time proclaimed, "The Giesl Oblong Ejector may be considered as the ultimate solution to the exhaust problem on steam locomotives combining, as it does, the best of theory and practice...". In 1963 a scheme was drawn up to convert a further twenty locomotives but by this time steam was already doomed and the project did not go ahead

The major differences between the WC/BBs and the MNs were that the light Pacifics had their cylinder bore reduced by 1.625 inches but retaining the same 24 inch piston stroke, the fire grate area was reduced by 10.25 square feet, a boiler barrel only 1.5 inches shorter but the diameter at the front being 3.75 inches smaller and the evaporative and superheating surfaces were smaller by 13% and 33% respectively. The boiler pressure remained the same at 280 psi though as with the MNs this was subsequently reduced. The overall wheelbase was 1 foot 3 inches shorter and the overall weight in full working order 5.25 tons lighter. Tractive effort at 85% pressure was 31, 000 lb, less than that of the Lord Nelson class but the boiler proved more capable of sustained performance albeit at the cost of being heavy on coal.

To meet the restricted Hastings line loading gauge the first 70 locomotives were built with a 8 ft 6 in wide cab however there were problems with forward visibility due to the narrow cab front window. A modification was devised altering the flat fronted cab to a wedge shape which allowed a larger window, and this modification was incorporated from new from the 64th locomotive built. Since the class was never used on the Hastings line from the 71st locomotive a 9ft wide cab was applied. All locomotives with a flat fronted cab were subsequently modified with the wedge shaped one.