Although ordered by the Southern Railway, the locos did not make their appearance until after nationalisation when 26 were built which became numbers 15211 to 15236. The locomotives, looking very similar to the Class 08, Class 09 and Class 11 shunters (and also those constructed by the GWR), had larger 4ft 6in diameter "Bulleid-Firth Brown" type wheels, (the same size as used by Maunsell on his 1937 diesel shunters) as opposed to the 4ft 2in, normally spoked variety used on the others.
 
There were few differences between individual locomotives, which at first were not fitted with the six-position marker lights at each end. Three of them, 15230/1/2 were equipped with train air brakes in the late 1960s whilst the remainder, like the Class 11s, had no train brakes. The Class 12s generally retained the ladders on each side of the radiator at their front end as the Souther Region had less overhead wiring than other regions and hence there was less danger to staff climbing up them.

Initially painted plain black with red bufferbeams, the locos were later painted Brunswick Green with the BR crest on the engine compartment door and at least one, 15212, was subsequently to appear in rail blue with the BR logo on the side of the cab beneath its number. At first the class was based in south London at either Hither Green or Norwood Junction. In the early 1950s some were moved to Eastleigh and later others moved to Ashford. During the 1960s some were to be found working from Bournemouth shed.

Bulleid's shunters had a central dual controlled desk, as in current practice, but in order to start the engine oil had first to be pumped up to engage the pressure switch. The pump was in the engine compartment which meant that starting required two people - one on the pump and one on the control handle engaging the start position. The locos lent themselves to easy maintenance as the exhaust valves were located in cages and could be extracted without removal of the cylinder head, as on the Class 08. Thus on the locos' six- and 12-monthly overhauls, the carbon build-up on the valves would be removed, the valves decarbonised, lapped in and then lapped into the cylinder head again without a major engine strip down.

Faults with this class were not that frequent, but one of the main occurrences was cracking of the cooling water transfer pipes, known as telephone pipes! The sanding gear also required regular attention, a common fault being blockage of the trap resulting in the air injector being unable to deliver sand to the rails, a fault more serious in a shunting locomotive than in a main line one because the rails it uses are in constant use. They were withdrawn from service between 1968 and 1971.