I hate to mention 'the elephant in the room' but the most effective and immediate way to reduce the amount of CO2 emissions might be, I am sorry to say, to reduce the total number of flights.
Saving a relatively small amount fuel by towing aircraft out to the runway may be pointless while there are plans to build a new terminal and a new runway at both Heathrow and Stansted and air travel is forecast to double by 2035 (or before).
While I know that there are many factors producing climate change I am beginning to be persuaded by the experts at the Met Office Hadley Centre who say that Aviation may be making a major contribution. As a professional pilot with many years of flying ahead of me, I hope they are wrong.
The huge Met Office computers nowadays produce extremely accurate upper air wind and temperature charts. The same computers also produce the climate change predictions. If we accept the former data, may be the latter data might have some validity.