Where and how can i get a decent room in london uk without needing a deposit?

A friend has only a few days to move out their place and has no money for a deposit. They can afford around �100 a week and is a very decent person. Obviously we are not naive and realise that in London you are not going to get the perfect place without paying through the nose. But in this dog eat dog world its about who you know and catching that opportunity when it arises, using whatever you have to attract the attention of anyone who may have what you need at that moment. So if anyone can help I would appreciate your advice and if were lucky an offer. to be clear, Looking for a safe, clean decent double room or self contained flat for around �100 a week, no deposit or monies required apart from the first weeks rent. For Female, Swedish. Thanks