I think you're a bit strict. While I agree that shots from Myrtle Avenue taken during midday in june or july don't look very pleasing, I still think it is possible to do some good shots in LHR even after early march.
I was there in early april last year and at least the light in afternoon (I was there from 4p.m. till 5 p.m. or so) was still pretty good. Perhaps not perfect, but still pretty good an I am sure most of us will see it like this, check the example here.
Otherwise I fully agree with Miguel, if you shoot in the mornings and in the late evenings, summer light can still be pretty good. That is what I recommended to a friend who wants to visit AMS in july. Wake up early, enjoy the early morning hours, pack at 10 enjoy the city, have a nice dinner and go back to the airport at 5-6 p.m. and enjoy the long evening.
Okay for example in FRA there is no really good morning spotl, as light will either come straight from the back or from the front, so it is not really worth waking up early (still the late summer evenings offer plenty of good spots), but in airports like Madrid, Amsterdam or Zurich this will work rather well.
Otherwise if there is anything very special coming during daytime in summer at my homeairport I try to shoot it very closely on the ground on areas with a light surface and with those reflections you can still avoid some of these awful dark shadows.
Surely not perfect, but I think it is still quite okay. Example taken on july 21st at 11:30 a.m.
Julien