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During WWI cars were donated by individuals for the war effort to be fitted with ambulance bodies. (Some wealthy families sometimes included their chauffeurs as well, the man joining the army). Some donors had ambulance bodies fitted with no consultation with the authorities, which resulted in horrendous problems because repairs and servicing were very difficult with no standard models. Eventually the British Red Cross laid down a specification for bodies and some form of standardisation was achieved which made it possible for ambulance bodies to be ordered direct from the manufacturers (See advertisements). Financial donations for ambulances were received from various organisations and businesses like Breweries, Trade Unions, charities, hunts, worshipful companies, and the Silver Thimble Fund.
Manufacturers of ambulances included Rolls-Royce, Daimler, Albion, Morris, Vulcan, Austin, Sunbeam-Rover, Wolseley, Siddeley-Deasy, Renault, Buick, Crossley, Vauxhall, Argyll, Sunbeam, Lanchester, Ford Model T, Fiat and Star

Roll's Royce advertised this vehicle in "The Autocar" in 1915.
http://www.vlib.us/medical/ambulnce/ambulnce.htm