This overfilling is pretty widespread across most nationalities just getting by in London, if you go by anecdotal evidence.
A friend of mine lives somewhere in North London (I believe it is Golders Green), paying 1300pounds a month for 2 and half room apartment, paying council tax and so on, all on a minimum wage as a porter which is around 1200 pounds a month(why he prefers being a porter over a Perl programmer that is one of his skills is another question).
If he were not subletting the room and half, there would be no way he would be able to work in London for a minimum wage. The situation is similar to most people working for minimum wage that he knows.
Still, I think this overfilling is a symptom not the cause of high property prices. This overfilling by the poor has always happened in big cities.