But what about the traffic above preventing climbs?
Then what are they doing there in the first place? All runways but Stanstead are East-West facing?
If you take Stanstead out of the equation, when winds are from the west all traffic arrives from the east and takes off towards the west.
So no one should be blocking anyone on climbout so why can't one sort out departure crossings (i.e. Luton southbound departures vs. Heathrow northbound departures) way up at 18,000ft+?
Or are you saying there could be easterly winds at Heathrow but westerly winds at London City?