Collin House (�50 per person, twin or double en suite, including breakfast).
Elizabeth (�49.00 per person, twin or double en suite, including breakfast).
Ebury House (�32.00 per person, twin or double en suite, including breakfast).
Vegas (�20.00 to �30.00 per person, twin or double en suite, including breakfast).
Enrico (�31.50 to �34.50 per person, twin or double en suite, including breakfast).
They're all close to Victoria Coach Station. For details (with customer reviews), and to book, go here:
http://www.hostelworld.com
(Don't be put off by the 'hostel' in that title. There are plenty of good budget hotels on that site as well). Use the 'Advanced Search Options' feature and put check marks alongside both 'Guest Houses' and 'Budget Hotels'.
Chris