I think what is becoming clear from this thread is that people are picking the mode of transport that is most convienient time and cost wise for them.
I live in West London so for heading up to the outskirts of manchester it is cheaper / quicker for me to take the car or the plane. But if I lived in central London and I needed to get to central manchester then the train would be better.
The politicians need to realise that people will tend to use what is more conveient for them so forcing those that live outside London to come into London to get to another UK city to then travel out again isn't going to work. All modes of transport need investment and then they might cut down on CO2 with people going direct to where they want to instead of going via city centres!!