Why is ARTCC room lighting in the US dim?
The FAA set out the ARTCC Facility Design Guidelines in this document. In short they are dark to minimize glare. 


  295. TRACON OPERATIONS ROOM. Lighting intensities in the TRACON operations room are usually kept at a very low ambient level, less
  than 5 foot-candles (fc), in order to prevent glare and reflection
  detracting from the target viewing on the radar displays. The design
  shall consider incandescent lighting with pinhole covers or framing
  capabilities in lieu of fluorescent lights. These incandescent lights
  can be aimed exactly where needed and, unlike fluorescent lights, can
  be dimmed to low light levels.


Its possible the British have no such requirement (under EASA or ICAO) or that the photo was simply over exposed and the room appears bright.