I've seen this arguement about domestic flights used a number of times - but does anybody have any evidence to support it e.g what proportion of pax to LHR transit? I don't use it as much as I used to but the NCL-LHR-NCL flghts I used to go where always packed, and just from watching where the rest of the pax went when they got off and converstions in the Exec lounge (which is where BA presumbly make their money) I wouldn't have said that anything like a majority were transiting.
For me, the train is only competitive if you're going city centre to city centre (and it's easier to park at NCL than the Central station).