Why all this talk about going on the observation decks?
At almost every airport you get better oppertunities from somewhere around the airport grounds, and proffesionals very rarely use Observation decks.
Just look at AMS, the observation deck is huge, but you need a 600mm+ lense to see 1
Runway, out of the 5, so there is really not much point. I beleive it is the same at LHR.
FRA maybe different but i am sure there are better places around the airport.
If i would go too one of these meetings then i would be at the end of a runway somewhere, not on the observation deck (obviously we'd meet in the airport