Why not just overcome the Hertfordshire boundary problem and extend the current runway
Most of the approach lights to runway 26 are located in Hertfordshire. By extending the runway a valley would need to be in-filled and the runway would still be partly in Hertfordshire.
Phase 2 expantion can only go south which means Terminal, runway and aprons inside Hertfordshire. This will be corrected when the County and town borders are updated and moved.
From me dated Oct 10th
Something radical is needed to get cars out of the central area not only for parking but also for picking up and dropping off passengers. One such radical idea would be to build a new terminal frontage in the mid term car park. Plenty of room here for a proper drop off area and short term parking. From here a rapid transit system would whisk passengers and meeters and greeters to the terminal proper in around 30 seconds
From the airports website
4) The new Terminal will be accessed via a new Reception Centre built on the site of the Airport's existing mid-term car park.
Ahh great minds think alike