The location of lights, including those to indicate the dimensions of vehicles, is set out in the Road Vehicles Lighting Regulations 1989.
In general vehicles must have front and rear position lights fitted within 400 mm of the edge of the vehicle. (For trailers, however, only those above a certain width are required to have front position lamps.) Larger vehicles must also be fitted with end outline marker lamps, and side marker lamps to indicate their width, length and height.