Puritan,
I don't think it is as easy as saying "cars are good, Aeroplanes are bad". I have only been trying to point out that your argument that a 737 is more fuel efficient than a car does not stand up and that you should not try to use it because it can be so easily turned around and used against you. That is all. Do you see?
Your other points: You are right. The "Greenie" argument I put forward could be advanced to the stage where we are all going around on donkeys which of course would be ludicrous. Equally though, if we are to extend your pro Aeroplane argument to its logical conclusion then we will all be flying in space shuttles from London to Edinburgh which would also be equally ridiculous.
It should by now be intuitively clear to most that given the choices that range from donkey to space shuttle there will be an option that is a lot faster than a donkey and a lot more fuel efficient than a space shuttle somewhere in between. My personal guess is that this would be the train rather than car or the aeroplane.
The focus of your argument should be that the lower fuel economy of aeroplanes is easily outweighed by the considerable benefits that aviation brings to the country. If you try to pretend that domestic aeroplanes are more economical than other forms of transport such as cars you will quickly lose the argument and that, depending upon who you are and who you are talking to, could result in higher taxes on aviation which we don't want.