Or Fly in 90 mins from Exeter or in 1:15 from Bristol. Not sure if Plymouth/Newquay have Scottish flights.
But TBH Air is probably the most suitable form for the distance and volumes. Train is the better option for journeys under 4hrs, which is probably why XC have geared their services to the shorter runs.
HS2 will dent London-Glasgow traffic when opened to birmingham, and will probably decimate it when fully open (I can see some flights operating as feeds for long hauls, but with smaller aircraft) - see Madrid to Barcelona (90% drop in air passengers between the cities) after the AVE opened. Similarly French domestic flights have plummeted with the LGVs opening, believe Toulouse is the only city left with a major air service (every hour) - don't believe the TGV has reached it yet...