In British slang, a jam butty is a police traffic or fast response car (as opposed to an ordinary panda car or patrol car).
The term came into common use in the 1970s, when such cars changed from the traditional blue and white colour scheme to white with a broad fluorescent orange stripe along the side. This colour scheme is reminiscent of jam sandwiched between two slices of white bread, hence the name. The slang was popularised on such TV shows as The Sweeney and Minder, as well as spreading through the use of CB radio. It is still in common use, although most forces now use different colour schemes with fluorescent stripes and Battenburg markings.