The phrase "cars" dates back to the Midland Railway and their Pullman vehicles which were known as "cars" and in BR days we used to refer to Kitchen Cars, Restaurant Cars, and Buffet Cars (RK, RFO, RMB, etc, etc)
DMUs and EMUs were not referred to formally as cars in language and documentation, except I believe, uniquely on the GE. Indeed Ilford Depot used to be known as "Ilford Car Sheds, and the signalbox bore that name also.
Outside the GE although we would refer to the formation of DMUs and EMUs as being x cars, we still referred to them as "units" or ECS. For example our depot shunter would say that he had an 8 car EMU to go to wherever, or we would warn a station that today an 8-car EMU was running as a 4-car or vice versa.