Why would you do that? The service from Newcastle is time critical v plane. Your plan simply means the train service is non competitive before we consider the economic impacts.
I think that this point is a fairly commonly-held misconception, and may even be in part the cause of some of the ECML's commercial woes.
Depending on service, around about 80% of BA's load to and from Newcastle will be connecting, not Origin & Destination (O&D) passengers.
With only six weekday services in each direction, as few as 180 air passengers per direction per day could be commencing and terminating at Newcastle.
O&D loads can be higher on the early southbound BA1321, as it almost guarantees being able to be in town for an 09:00 start. That currently is an 06:05 from Newcastle. By rail, that would be an 04:45 or, at a push, 05:25 from Newcastle.
The up Scotsman is the only service that VTEC really seems to want to market to a 'time-critical' audience, but then that doesn't reach King's Cross until 09:40, making 10:00 meetings a push.
In any event, I suspect that
TBY-Paul was referring to services which already take up 43 minutes longer than the Scotsman as being less time-critical.
Ultimately, it's frequency and reliability, as well as intermediate pick-ups that are the railway's strongest card. Attempting to conquest from air between Newcastle and London is largely pointless.
The situation ex Edinburgh is quite different, despite the longer rail journey time, as O&D volumes and proportions are higher between there and London, meaning that the railway has more to play for. Rail operators have, of course, long known this and offered aggressively-priced products that are not available on shorter flows.