The under runway tunnels at Heathrow are undergoing a 2 year refurbishment that started in 2015.
For security reasons GSV doesn't show the central terminal area including the tunnels, but a good video showing the old fluorescent and SON lighting is here.
LED lighting is now installed in both tunnels where the old lighting was. It is in the form of linear arrays of LEDs.
This internet image appears to show the new lighting but at full power mode, during the daytime the LED modules are more distinct spots - almost like a single LED and much dimmer. Only alternate sections are lit - as in the left foreground - although it looks like the lighting can be continuous - see distance. The lighting on the right tunnel wall looks to be temporary.
Meanwhile on the northern perimeter road, some GEC SOX bricks still survive and are working. However the lighting is in poor condition with large sections of lighting (including SON sections) which are no longer working.