you are burying your head in the sand if you think LHR isn't the primary cause of traffic in and around the M4/M25 interchange.
Quite wrong, I'm afraid. LHR handles around 70m passengers per year - that's just under 200,000 per day. of those at least half use public transport to get in/out of the airport, so 100,000 per day. Let's assume the average group is 1.5 people (families, couples, business people travelling together), so that's 67,667 car journeys. Of those, perhaps a third will use other routes (into London on M4, north/south into suburbia, A30, etc), so 50,000 per day. Spread over perhaps 18 hours a day - 2,777 cars an hour.
Then add those who commute into the airport. 76,500 employees. Again, some will use public transport, they will come from all directions, and their shift times will cover at least 18 hours a day. In total, perhaps the same number as passengers. Sum total of cars using M4/M25 junction, something over 5,000 cars per hour. My generous estimates compare with a capacity of over 20,000 vehicles per hour on a 4-lane motorway.
If you look at a map
Dobbo Dobbo you will see that both the M4 and M25 service some of the most densely-populated and affluent parts of the UK, so your assertion from Leeds is about as far from the truth.