Hotel for £75/night for 3 in London..?

My father and two sisters are staying in London for four days in early July.

They don't want anything with frills, just clean with the basic living facilities and below £75/night.
Preferably as central as possible - they will be using public transport to get around.

Any suggestions?

Forgot to add, my sisters are 16 and 19. If that makes any difference.

travelodge.co.uk
premiertravelinn.co.uk

Family room at Travelodge. Dad can sleep on the sofabed

Check out unversity accommodation - many do twin rooms. They are usually well located, have self catering kitchens and are cheaper than hotel rooms. I know the LSE and Queen Mary both rent out tourist accommodation in the summer, the rest probably do.

Thanks a lot! They are weighing up these two options now. :D