Hi,

Here is my approach:

1. Take all London airports (5)
2. Make an assumption on how many landing / departing lanes each has (2 for the big ones).
3. Assume how frequently the planes are landing / departing (based on your personal experience and what you've observed while traveling). For small ones the frequency may be as high as for big ones since they have a lot of discounters who spend just 1-2 hours in the airport
4. Take into account peaks and off-peak times (fewer flights late evening).

Sanity check - how many flights you see on the screen for a 3-4 hour period when yu come to the airport

Best!