ornithopter raises a good point about the flexibility of transfers. Stansted rail link is a joke. I live in W London, only 10 miles from Heathrow, and it takes over 2 and a half hours to get to stansted(thats before i check in!!)
It takes me 15 minutes to drive to Heathrow, but god forbid I should take public transport....it takes at least an hour on the tube, and the longest it ever took me to get home was 1hr 45 minutes from terminal 1 via the Piccadilly line/district line.
Gatwick is the big surprise. It reliably takes 50 minutes at most times of the day by car, but can take up to 2 hours by Public transport, mainly spent stuck on the way to Victoria station, from where the service is usually pretty decent(and cheap)
My vote is for Northolt to become a domestic only,with a high speed train/road tunnel link to Heathrow. The road tunnel would be used to transport secure checked in baggage to foreign flights, and the train for the pax.