I had a stay at the Regents Park Marriott a few years ago and really enjoyed the property. Not a gigantic hotel and located in a residental area (Regents Park/Zoo) so did not have the " big city hotel" feel.
Nearest tube station Swiss Cottage a short walk away. Also great little nieghbourhood restaurant next to the school (on the way to the subway station) name slips me right now.
Mike