Hi PAXboy.
Northolt arrivals through the airways system are already worked by Heathrow Approach. They are released into each of the stacks in the same way as LL traffic and then woven through the LL traffic into a downwind position before handover to WU director. Given that they use LLs airspace they have no impact on the STN, LTN, LCY operations. To many intents and purposes, WU is already a LHR satelite.
However, that all works on a rate of 6 per hour at which it is easy enough to get them released into the bottom of the stack (no "drop through" to be done). Twenty per hour would be more difficult and would require some jigging of the procedures but is unlikely to be as difficult as STN would be.
Heathrow/Northolt has the airspace, Stansted doesn't.
Point 4
