In what is sometimes refered to as 'The Triangle of Death', that volume of sky between London, Luton & Stansted, the base of CAS is 2500'.
Various spot heights in N London are about 360', so to make life easier, assume ground level to be 400'. Add on 1500 which gives 1900' and subtract that from 2500'. This gives a sliver of sky 600' thick max. to fly in.
For the purpose of a prosecution, the CAA's definition of a settlement is 'two houses or more', so going much lower than 1900' QNH isnt a good idea, unless you want to weave around the houses.
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.
That extra 500 feet has just doubled the volume of airspace available.