Get the train.
The GNER service (London-Edinburgh, up the East Coast Main Line) is only four hours if you get the fastest service and the trains are very comfortable, especially the ones that have had a refurb recently. Cheap too, if you book well ahead.
Already, better long-distance train services are taking custom from the airlines on London-Manchester and London-Scotland services, and if they could be speeded up further (sadly unlikely, in the near future) then internal flights really would contract. After all, once you've got to the airport, gone through security, flown, got through the airport on the other end and managed to get from a field in the middle of nowhere to where you actually want to go, you might as well have taken the train anyway.