Try the Classic Hotel:
�45.00 per night single, en suite, including breakfast, close to Lancaster Gate tube station.
Or the Balham Lodge guest house:
�38.50 per night single, en suite, including continental breakfast, close to Balham tube station.
Or Tony's House Hotel:
�27.00 per night single, shared bathroom, including continental breakfasr, close to Paddington tube station.
Or the Ebury House Hotel:
�30 to �35 per night single, ensuite, (or �25 to �30 per night with a shared bathroom) but no breakfast, close to Sloane Square tube station.
Or the King's Hotel:
�30 per night single, shared bathroom, including breakfast, close to Kings Cross-St Pancras tube station.
Or the Metro Hotel:
�35 per night single, shared bathroom, continental breakfast included, close to Lancaster Gate tube station.
All with generally good reviews here:
http://www.hostelworld.com
(Use 'Additional search options to select 'Guesthouses' and 'Budget hotels').
Chris