Registration numbers for bikes? Never: it would cost too much to administer, would drastically reduce cycling levels, and wouldn't reduce the minor problem of yob cycling.
(The yobs would simply ignore the law, just as many yob drivers do - some estimate that 1 in 5 London drivers are uninsured.)
In some ways it's a shame, as it would be nice to have personalised bike registration plates: C7CLE, or B1KER, or (for Dutch fans) F1ETS, and so on.
(It's amazing how the number plates of vehicles blocking cycle lanes that I photograph follow a pattern. It's almost like someone had Photoshopped them.... see here, here, here, here, here, here, here, here and here.)
Curiously, the number plate on the car in this photo, snapped last week (and parked legally, of course) is genuine. You couldn't make it up.