Sorry I can't offer any particular recommendation, though I know the ibis Euston St Pancras is fine and at least it is convenient.

More generally, if nothing suitable comes up with the bigger online agenst, I usually try one of the cheaper brands including Travelodge, Premier Inn, and the Accor group (e.g. Ibis Budget). I also recommend looking at University Rooms for single travellers looking for availability through holiday periods. Also for cheaper B&B style accommodation in the UK that don't book through the bigger agents, try laterooms.com, hostelworld.com (which has filters for private en-suite rooms) or a bed and breakfast directory, but try and find some reviews before booking(!)