All London hotels are rip-off including Holiday Inn or Premier Inn but, if you look at hostels or shared accommodation you may find a reasonable bed for not too much £.
Hootananny in Brixton will do beds at around £14 per night:http://www.hostels.com/hostels/london/hootananny-hostel/24068
Hootananny Hostel, Brixton
95 Effra Road
Tel: 020 773 77273
But I believe these cheap beds would have to be booked very early in advance!?