Low Cost Hostels. These can be booked online. Would suit those who can present as workers, visitors or tourists.
http://www.northfieldshostel.com/
http://www.hostelworld.com
http://www.hostelbookers.com/hostels/england/london/
http://www.hostels.com/london/england
http://www.budgetplaces.com/london/budget-hostels-list/
http://www.travelstay.com/Cheap_London_Hostels.htm
Advice on getting Free Accommodation in London:
http://www.brokeinlondon.com/free-accommodation-london/