This is the easiest hotel recommendation I've ever written. I cannot recommend this hotel enough. The location is great; a stone's throw to a number of shops, markets, and restaurants, and a short walk to the Circle Line. It's also a ten minute walk to Portobello Road.
The hotel itself is lovely; the rooms are spacious, clean, and comfortable. Where 55 really shines is in their high service level and wonderful staff. From check-in to check-out, everyone we interacted with was helpful and gregarious.