I'm sure that I've heard in the past that every arrival at Northolt is considered by ATC as a Heathrow arrival due to the proximity and separation requirements.
If that is the case, it surely puts the kibosh on a significant operation in/out of South Ruislip regional? Perhaps a curved RNP-AR approach from the north might be an enabler? Though whether the Q400 is so equipped, I'm not sure. Or whether anybody of any authority might make a decision to pony up the cash to do such a thing...