The difference is that Heathrow's proposal applied to entering the airport, and HAL would get the money, whereas Boris's proposal to extend the London Congestion Charge means that his own Transport for London would get the money
I really do get fed up with "green" schemes which are nothing more than a money grab by the proposers. They actually hope the number of cars is not reduced, as otherwise their income for their shareholders (HAL) or the staff's gold-plated pension fund (TfL) is reduced.