Looks like you booked already but here are some that I use :
Holland House Hostel, Holland Walk, High St Ken Tube Station, 0870 770 5866 : beautiful location in a park, about £20 / night dorm room
Hampstead Heath YHA, £22/night, Golders Green Tube Station, 4 Wellgarth Road, 0870 770 5846 : beautiful area north london away from tourists
Generator, 073887666, £15ish, Russell Sq, 37 Tavistock Pl : central london, busy for young people who want to hang out with other young people ! !
International Student House, 229 Great Portland Street, London W1, Tube Great Portland Street Bus C2, 88, 453, Tel (020) 7631 8300 x 1
Website www.ish.org.uk, £34 single