I do the trip from Edinburgh to SW London regularly too. On average, the train trip is 30 mins longer door to door. Public transport at the London end is crap for both methods, at Edinburgh it's a 10min bus ride or a £6.00 cab fare.
Prices tend to be similar unless you book months ahead, where the train wins.
But at least on the train you can work, relax, walk about and get food. And the scenery can be beautiful.
So you pay your money and you take your choice.
The airports and the train stations are equally unpleasant.