re: The UK phenomenon
No question, they're fascinating thingies and whoever makes them has keen artisitic sense. I expect--though I obviously could be wrong--that they're designed and executed by something of a subculture perhaps not unlike some of the more talented graffiti artists who adorn the sides of our trains (and maybe subculture is too strong a word).
I suppose they can't really come forward without risking charges of trespassing and property damage...but it'd be interesting to find out what kind of people are involved. They certainly deserve an "A" in secrecy.