That being said, the idea that the Central line uses some kind of variant indicator is not a bad one, but I happen to know the Central's RGI's are the same as everywhere else's.
Key Protection is an excellent idea, although I have the feeling I've seen them on Video125's DEV in places where Key Protection Switches seem to make less immediate sense, but that's not to rule it out - not least because I'd have to watch the video again and note where else they appear.
It's for traction current section switches, so if a section switch is operated and traction turned off then they will light up. So for Marble Arch, you can isolate the siding from the mainline or if the mainline was knackered you could isolate that and still use the siding.
There is a traditional rail gap indicator just before these lights by the station starting signal.