So this means all the east-west traffic to the north of London gets squeezed into a slice of airspace 500-600 ' deep and I would imagine that the same conditions apply to the south, between London & Gatwick too.
Whilst you can't fault the analysis, the fact is that most traffic operating VFR under the London TMA manages to select 2400 QNH to fly-at without any form of regulatory intervention. The revelation that there is a slice 500-600' thick would actually be a breath of fresh air.
2D