Just about all the public street lights in my area are sodium, although metal halide are becoming more common in privately owned areas, e.g. supermarket parking lots.
Now I come to think of it, it must be many years since I've seen a mercury-vapor type of light. I've lived in rural areas a long time now and avoid big cities whenever I can, and as I recally the mercury lamps were most often found in urban areas.
I'm not sure that series street lighting was ever employed in Britain. Most is just run straight off the 3-phase distribution lines with either timer or photocell control. Some older installations used a fifth pilot wire to close contactors at each light.
