Wrote :- "We've already reached saturation of interior lighting levels in our homes (at least in the UK)
So all homes have all their lights on all the time?
Offices, yes. At work there is a glass covered (roof and sides down to knee level) 50 yd link between two buildings. Every morning its continuous line of flourescent tube lights is still on from the previous night (presumably the night guard patrol turns it on in summer). I turn it off each morning, because it is as bright as daylight in there. If I do not, no-one else does. In fact if anyone else sees me turn it off they give me a very funny look - almost makes me ashamed to do it.
These people would not leave such lights on at home - yet. But even I now keep my hall light on all evening because it has a low energy bulb. I did not leave it on when it was incandescent, so I certainly use more electricity there now than before.