The Slingshot is unique

 

The Polaris Slingshot is a three-wheeled motorcycle. The manufacturer, Polaris, makes it very clear that it is not an automobile. And because it is not an automobile, it does not have airbags nor meet many other automotive safety standards. 

Both the driver and passenger, must always wear a DOT-approved full-face helmet and fastened seatbelts. And since the Slingshot is classified as a motorcycle, a driver's licence for motorcycle is also required.

According to Wikipedia:

The Slingshot is made by Polaris Industries in Alabama. It has a tilt-adjustable steering wheel, side-by-side bucket seats, no roof, doors, or side windows. The interior is waterproof so it can be hosed down and the water drained out via drain holes in the floor.

A specialty motorcycle shop in London sells the Slingshot and with this local availability these are not uncommon on London streets.