If I were to do it:
Heathrow is one of if not the most busy airport int he world, therefore assume a plane leaves/arrives every 10 mins. That would be 6 planes an hour and with 2 runways makes 12 planes an hour total. Which equates to 288 planes a day (12*24).
Understand this may be a high range value as there will be inefficiencies in some planes (go arounds etc.) and peak/off peak times.
Then I would take an average plane of 400 passengers and times this by 250 planes per day (moved down from 288 to account for inefficiences and off peak times - could be lower).
This would give 100000 passengers per day at heathrow from arrivals and departures.
Interested to see how other people would do it (probably could do it on number of gates at heathrow and a plane leaving/arriving every hour or so at a gate)?