I don't know about London, but in lots of these places, occupancy is determined on the honor system. They don't check power/water consumption or lights on at night like other places. In china, you just need to count the lights on at night to see how big the problem is, or peep through the door to see that the unit is still not even renovated. They did a study a while back using power consumption stastistics that was quickly hushed up as a state secret.

In Vancouver, the problem is so bad that local shops are going out of business for popular speculative properties because their just isn't enough residents to use them, even though prices are sky high and the developments are all bought out.