Slang for
squad cars in the
UK which are often painted with a red stripe down the side (hence, the jam part of the sandwich).
This signifies the car is not a normal traffic patrol car, and that the car is carrying Armed Response Vehicle Officers. Generally these cars would not be out on normal patrol, but instead responding to a request for reinforcements to deal with an armed and/or dangerous perp. If one of these comes for you, consider yourself well and truly nicked.
Keep in mind this distinction must be made since many police officers in the UK are not equipped with firearms, unlike, say, North America, where all cops have semi-automatic handguns and a shotgun in the front seat. Which makes it all the more confusing that every customer service window in the UK has bulletproof gas, while banks in Canada are often completely open counters.