It's unwise to say that rail is always better than other forms of transport, and it's equally as unwise to say that flying is always better than rail. Although Paul1609 didn't say exactly that, just that rail wasn't viable competition, it's as near as it can possibly be in my book!
It's all down to individual choice, and most people will consider different things when they come to make a decision. That will include the frequency of services, comfort, speed, cost, convenience, accessibility and may include such things as fear of one particular mode of transport as well.
For many, flying will be their decision on a route from the south of England to Glasgow. For others, a train via London and either the west or east coast works. Some might choose to go via Southampton, Reading and Birmingham to avoid the cross London journey. A few might find the route via Bristol a better choice, though I wouldn't go that way myself.
The point is that there are plenty of factors to be considered before making the choice, and it's usually a balance or compromise between them that leads to a final decision to be made.