Here are a few random prices (for a single room, midweek);
St Athans B&B (near the British Museum): £46
City Hotel Carlton (Victoria): £64
Kings Hotel (Kings Cross): £30
Mowbray Court Hotel (Earls Court): £45
St George Hotel (Paddington): £45
All from
http://www.hostelworld.com
(Don't be put off by the word 'hostel' in that URL. Simply carry out an initial search and then use the panel on the left hand side to deselect 'hostels', while adding in 'hotels' and 'guesthouses').
Chris