As a private pilot based at Panshanger, I can confirm that you don't need to squark squat to transit north between Luton and Stanstead, as long as you are below 2,500ft. The airspace you see on the 1:500,000 map is effectively a bridge connecting the two TMZ's ranging from 2,500ft-3,500ft. It's all class G below that. You obviously have the listening squark code for Luton and Stanstead (0013) which is useful if you are unfamiliar with the area.