If your criteria of "close to the city centre" is flexible, then I would recommend a favourite of mine - the Hilton at Wembley. It is close the tube station, which in turn is just two stops from Baker Street on the Metropolitan Line. TBH in terms of time it's no further out of the city than Greenwich or the Excel. It tends to be much cheaper than hotels closer to the centre for both cash and points redemption, has a great lounge and restaurant breakfast, a pool, lots of restaurants nearby, and they have family rooms with 2 x king beds.