I have gotten a rate in the low 100 pounds at Forte Posthouses. The one in Bloomsbury is in a somewhat dull neighborhood but is a short walk to Soho, etc. and tube stops. Also stayed in Kensington which is a nicer neighborhood but much further away from evertying. Preferred the Bloomsbury location.

Posthouse is roughly equivalent to a Doubletree in terms of quality.

You might try hotelreservationnetwork.com too.