I'd suggest, perhaps, The Indigo chain. They are part of the Holiday Inn Group but in London (Earls Court (referred to as Indigo Kensington) and Paddington) they are boutique in nature and very good.

Too many bad stays at thistle hotels in the past so am reluctant to go back.

Other recommendations - Doubletree Westminster (used to be the old City Inn) and Park Plaza Riverbank, although the location of if the PP is not ideal