In essence you can't travel direct due to the rail strike.
There are no Avanti trains running.
You will have to take a Scotrail service from Glasgow Queen Street (fastest) or Central to Edinburgh Waverley, then an LNER bus to Newcastle then LNER train to London Kings Cross.
A few LNER trains are running through from Edinburgh diverted via Carlisle.
Or take a Scotrail Glasgow to Carlisle train via Kilmarnock and change into a diverted LNER service there- but no easy route
EG- Glasgow Central depart 1113, Carlisle arrive 1342, depart 1430 Newcastle arrive 1545 (change trains), depart 1600, London Kings Cross arrive 1853
EG- Glasgow Queen Street depart 1100 Edinburgh Waverley arrive 1144, depart 1215 Berwick on Tweed arrive 1350 (Rail replacement coach), depart 1415 (rr c oach), Newcastle arrive 1605, depart 1758 London KC arrive 2131.
or- depart Edinburgh 1252 (train via Carlisle) Newcastle arrive 1545, depart 1600 as above.