Thanks! Now I understand what that panel of light really is - I though it was an illuminated ceiling!
Walking through the Elephant and Castle area of London as a child, this is the building that first made me realise that modern buildings can be exciting. It had a silvery finish and no windows, which made it unlike anything I'd ever seen before:-
It was only by writing this post that I learned that it is an electrical substation (hence no windows) and includes a neat bit of design that can only be appreciated from above:-
( In case the two pictures don't make sense, one of the four black legs in the bottom picture is seen on the left-hand side in the top picture, ok?)