Look at the door to door times and any additional costs, you might be able to get a bus to the station but need a taxi to the airport or vv.
London to Newcastle is mostly by train but London to Glasgow is mostly by air. A VIP coach with a business class style seat is more comfortable than economy on a low cost airline and from the city centre to another city centre might be cheaper and quicker, especially if the drop off point is across the road from your hotel.