Here's my take
- Improve rail links into LHR with AirTrack and better connectivity from the west and northwest.
- Make LHR rebalance its landing fees so that smaller planes are further dissuaded from flying there.
- Create dedicated access and transitways for ground shuttles between LHR and LTN, if not a full blown light/heavy rail link
Edit: SEN's catchment is 1/3 English Channel...