I know the argument of even a little reduction for cars helps for the airport but how much are you really saving really in pollution for not including Heathrow? They are clearly not doing it because it's an airport, it's for cars, that much is clear, if it's for the airport then ULEZ will be applied in all airport.
The part that is funny to me is because it punishes people who don't live in London but need to go to the airport, your alternatives....often i like to get an early flight because it works out better in both time zones and price. Is there a train leaving my house at 1am? No....the alternative to get to the airport earlier would get the train a day before, pay to stay close to the airport. The last time I took the train at the airport, due to one thing or another I miss a connection on the way back and had to buy a new ticket. It also takes like 5hrs on the train vs about 2hrs 30 driving.
Do you call that an alternative, sure, it is an alternative, its hardly an attractive one, it's not even close, it's not an even alternative, it's juts above walking considering how bad Network Rail is with so many strikes and cancellations and delays.
The other alternative is the coach service, which means it will limit my choice of flights as they are hardly running every hour, more like twice a day....and it's not even direct, it requires a connection change for the bus.
There are alternatives to cars....but would you really? In my shoes?