So what’s the deal? It is a real antique car or a mock-up made for a movie?
IIRC, there were multiple CCBB prop cars. Although most of them lacked any sort of engine, one of the cars was built with a butane-powered engine to allow it to be driven both indoors and outdoors. While the car was built to be a movie prop, and would likely not be durable enough for use as an everyday automobile, it is (or would be, but for H.M. government bureaucrats) a drivable which, given its age, could reasonably be called antique.