There are pages which show the busiest routes (Like here: https://en.wikipedia...ger_air_routes)and this should be reflected in game. This says that the London - JFK demand is around 8 thousand people PER DAY roughly where as the game says demand is around 1200.
Keep in mind that the stated daily demand of 1200 is for the green (ie, full-fare) passengers only. Perhaps the remainder of the passengers would be the half-fare-price CONNECTING passengers. So maybe - just maybe - you can happily put enough seats on the routes, i.e. you simply fly 8000 seats, 1200 filled with full-fare passengers, the remainder filled with the stand-by (or bumped) passengers paying half-price..
Also noteworthy is that in this game the London - JFK route's daily demand also has C- and F-class passengers. Convert each C- and each F-class passenger to the equivalent economy passenger count and add that amount to the total. Perhaps that might explain part or most of the gap between your 8000 figure and the 1200 stated economy passenger figure. Or it might not.
Or perhaps a combination of the above 2 possible explanations is at work.
One final idea - have you considered the fact that maybe in the starting year, 1200 daily was the correct total figure in the first place.
You might mistakenly be using 2016 figures for the year in question.
The point is this - someone will always be able to claim rightly or wrongly that some daily demand figure is right or wrong.
I recommend that you just play the game as-is and simply accept the figures for what they are - rough estimates and nothing more.