There are things call logic, it’s illogical to pick and choose tax certain pollution and not others, it’s illogical to tax a car and then not do anything about the airport. It is also a sense of frustration to tax people who doesn’t live in London and not going into London. The line is arbitrary, it’s drawn by human and can be redrawn again by human.
Heathrow is a massive hub for the whole country, there are people who lives miles away that needs to go there, it just happens the airport is located inside the M25, and they are affected by it. They can’t turn back time and move the airport, but they could easily exclude it from ULEZ as I’ve drawn though. It doesn’t affect ULEZ in London, as people going to the airport isn’t going to London.