From 2001, several operators ordered multiple units with different types of power plant for use in Great Britain, which are combined by Siemens under the brand name Desiro UK and are adapted to the British loading gauge. They all reach a speed of 100 mph or more. In addition to a total of five electric series, there is the diesel-hydraulic class 185, which is described separately. It started with the multiple units for South Western Railways, which are used on the extensive 750 volt DC network in London and the areas south-west.

South Western's 2001 order was for a total of 785 cars, divided into three different variants, all with contact shoes for the third rail. These were 100 four-car Class 450/0 trains for outer suburban routes, 32 five-car Class 450/2 trains for inner-suburban routes and 45 five-car Class 444 express trains. As the existing platforms were too short for the five-car trains, the order for the 450/2 was cancelled. The order was converted to 30 four-car class 350/1 multiple units for Central Trains and ten additional 450/0s for South Western. The latter later received 17 more Class 450/0 trains. The 45 sets of class 444 are mainly used between London Waterloo and Portsmouth and their buffet car has a recess on the roof in which pantographs could be used later if necessary. These trains were built by Siemens in Vienna, all others in the former Uerdingen wagon factory.